Treason in Arthurian legend

The concept of treason can be dated back to the early Roman republic, but was defined by nebulous criteria. Frederic William Maitland, author of The History of English Law Before the Time of Edward I, has said that "treason is a crime [with] a vague circumference and more than one centre." Early French and Anglo-Saxon laws for the prosecution of persons deemed traitorous were inspired by, and in some cases, directly pulled from, late Roman and Germanic conceptions of the crime. It would be the common laws of this time period which would most directly influence those customary in King Arthur's court — assuming its existence is founded in more than the legends and fables of medieval romances.
The origins of the word treason, as speakers of English would recognize it, date to the 13th century. Indeed, prior to the Treason Act of 1351, there were few laws which outlined comprehensively the legal qualifications for treason, or the appropriate punishments for said crimes. The process of conviction, trial, and sentencing of traitors such as Lancelot or Mordred in Arthurian literature was greatly informed by the very real, very gruesome practices of both French and English courts, depending on the geopolitical origin of the text's author.
Pre-Norman Conquest Conceptions of Treason.
Roman Definitions of High Treason.
While it is difficult to pinpoint precisely where the concept of treason originated, as it presumably is as old as history itself, the earliest legal documents pertaining to it are of Roman writ. Its first Latin manifestation came in the form of perduellio, which etymologically stems from the Latin terminology for "base enemy," or "wicked warfare." Thomas R. Robinson of the Georgetown law journal defines perduellio as referring to an enemy to the state "devoid of the virtues of a [foreign] enemy; openness and frankness; a treacherous enemy from within." He also writes that "it is safe to assert that perduellio was, in the beginning, largely an offence against the military power [of Rome]." While its etymological denotations would imply that perduellio was reserved strictly for military offenses, Elmer Truesdell Merrill states in Cases of Treason in the Roman Commonwealth that it is
Towards the end of the Republic, perduellio was phased out of its common usage, in favor of a term which, in the words of Merrill, "did not seem to suggest the idea of military delinquency as its essential element of applicability." Crimen maiestatis, or "majestic offenses", became the general-purpose term for all acts falling under the category of treason. Following the rise of crimen maiestatis in popular use, perduellio was retrospectively relegated to solely military insurrections and rebellions. Maiestatis literally refers to the supreme status and dignity of the nation, but was more commonly used to refer to the supreme dignitary of the state, his majesty the emperor. This informed the later middle age interpretations of treason as offenses specifically contra the royal family.
Two laws in particular, lex Julia Maiestatis and lex Quisquis, served to frame the general definitions of treason not only in Rome, but in early medieval Europe, centuries later. lex Julia Maiestatis defined the following as treason:
Lex Quisquis appended the previous statute to include assassination of the emperors councillors, failing to reveal or report a treasonous plot, and perhaps most importantly, the cogitation of treasonous acts against the crown, or "cognitaverit contra animam regis," as acts of treason. It is "best known," however, "for its provisions on punishment." 
Lex Quisquis most directly influenced early French statues on treason, but both lex Julia Maiestatis and lex Quisquis were borrowed from heavily in the formation of the English Statute of Treasons of 1352.
Germanic Definitions of High Treason.
The British Isles maintained, historically, some level of isolation and autonomy from mainland European powers. While waves of Romanism washed across much of continental Europe, Britain stood resolute. Therefore, "the laws of the anglo-saxons were affected by this process more slowly than those of most other Germanic peoples." Moreover, "what Roman influence there was," claims John G. Bellamy in his book, The Law of Treason in England in the Later Middle Ages, "may have [only] been conveyed to England through the Church." This left early Anglo-Saxon perceptions of treason to very closely mimic greater Germanic doctrine. In contrast to the Roman axiom of holding the emperor above all, as a physical embodiment of the dignity of the nation, pure Germanic treason was defined by loyalty to one's lord — with "no special sanctions against hostility directed towards the king." In early Germanic cultures, "[a] King lived and ruled under the laws of his people." The dignity of the state, therefore, would truly mean the dignity of the res publica. Treason consequently took on a very personal definition, in the form of treubruch, or a breach of faith or trust.
Ironically, by the fall of the Roman Empire in Europe, circa 500 AD, Britain could no longer withstand bending to Roman influence. Using treubruch to describe treason was falling out of fashion, with laws barring the general violation of strictly royal dignity taking its place. Treason was also, possibly for the first time, being divided into categories of high and petty treason. The Code of Alfred, otherwise known as the Doom Book, was a compilation of Saxon laws written circa 900 AD which differentiated high treason from petty treason as offenses committed directly against the king, versus those committed against one's immediate lord. Forgery of national currency was also recognized for the first time as treason, although its classification was fluid, as the severity of the crime was determined by the scope of the act's negative influence.
Post-Norman Conquest Conceptions of Treason.
French Definitions of High Treason.
French Royal officials adopted the Roman laws of lex Julia maiestatis and lex Quisquis. They defined the highest form of treason as Regicide, or the murder of a king. Second to the actual assassination of a monarch were attempts to depose him. Plots to overthrow the king, deposition, and conspiracy to "replace him at the helm of government," were all defined as high treason. Although threats against the king were considered most grave in nature, those made against the royal family were also serious offenses. Causing harm to any members of the royal family also constituted treason. Beyond the self-evident treasonous crimes of murder, injury, and conspiracy, were more vaguely defined offenses involving the royal family. Those implicated in scandals surrounding members of the royal family could also be accused of treason. In 1314 "Philippe and Gauthier Aulnay, minor nobles of Philippe IV's household, were executed as 'the worst kind of traitors' for their adulterous relationships with the king's daughters-in-law."
English Definitions of High Treason.
Later medieval definitions of English treason were derived primarily from antiquated Germanic and Roman sources. As in the Roman statute of lex Quisquis, failing to reveal a treasonous plot was considered, and therefore punished, as complicity in said plot; also, those suspected of plotting treason would be tried as if said plot had been carried out, and sentenced to the same punishment as one guilty of the actual act. The English also adopted the French concept of cas royeaux, defining treason as a general violation of royal dignity. Through the Magna Carta, however, "English kings [were] prevented [from] becoming theocratic monarchs by means of Baronic cohesion." This meant, at least legally, that barons were provided (albeit limited) forum to raise grievances against the crown, in the name of the defense of their own collective rights as barons. Magna carta was signed in 1215, and was swiftly annulled, but nevertheless had long-lasting implications on English common law. England's first "official" statute outlying various legal definitions of treason came from the Treason Act of 1351, and subsequent English Statute of Treasons of 1352. Both served to codify a collection of various crimes under a general heading of treason, and provided appropriate, standardized punishments for said offenses. The statute was also impactful through its creation of new types of state trial, designed specifically for alleged traitors.
There were typically three parties involved in English treason trials: an "actor," or plaintiff, a defendant, and a judge. This three party construction was considered "one of the cardinal rules of English law...in matters of jurisdiction." In cases of lese-majesty, or injury committed against the king, the king was automatically considered the plaintiff and thus could not act as the judge in a treason trial. Instead, a judicial council was selected to determine if the offense in question constituted treason. English treason law also consisted of complex laws concerning the treatment of accessories to treasonous crimes. Until the principal offender in a treason case had been convicted, "an accessory of an alleged crime could not be proceeded against." If both an accessory and a principle tried to flee, then both would be considered guilty. However, if only the principle attempted to evade arrest, then proceedings against the accessory could not commence until such time as the principle was apprehended.
Treason and knighthood.
Knightly treason was seen, naturally, as a betrayal of a knight's chivalric code. By its nature, treason betrays the chivalric values of loyalty and brotherhood. The status of knighthood was fiercely defended as one of true nobility and manhood. Therefore, knights who committed treasonous crimes threatened to undermine the sanctity of the order as a whole, and all that it stood for. Thus, punishments for knights and nobles deemed traitors typically involved some kind of process in which their identities as both knights, and men, were stripped away. Executions of treasonous knights included a process of removing the physical representations of their titles, such as "breaking of the traitor's sword and striking of his spurs, removing his sword belt, and publicly stripping him of the rich clothing that visually communicated his status." Punishments could also include an either literal or symbolic emasculation of the accused, through dismemberment or disembowelment. These punishments "feminized" the traitor, thus separating him from the ranks of "true knights" and "true men," implying that his status as a traitor rendered him a "false" version of both. By severing those convicted of treason from their identities as knights and men, they thus preserved the "elite masculine identity" of knighthood. 
Treason in Sir Thomas Malory's time.
Sir Thomas Malory, author and compiler of Le Morte d’Arthur, the preeminent compilation of Arthuriana of its time, was writing during a time of great political upheaval in Britain. As Robert Kelly writes in his article "Malory and the Common Law", "treason cases were never more politicized and controversial in the late medieval period than they were in Malory's time during the conflict between the Lancastrians and the Yorkists..." The War of the Roses was a series of sporadic political conflicts in Britain for the throne, spanning from 1455 to 1487. The war was fought between two rival factions of the House of Plantagenet — Lancaster and York — for ultimate control of the crown, and was ended when a Lancaster, Henry Tudor, defeated Yorkist king Richard the III at the Battle of Bosworth Field in 1485, though there were several smaller political skirmishes afterwards. During the War of the Roses, accusations of treason were frequent and often ill-founded. A provision in the Treason Statute of 1352 stated that an act of treason only had to be imagined to be treasonous. Because it was difficult to determine whether or not a person was imagining treason, words spoken against the current king were taken to be indicative of traitorous thoughts. Similarly, words spoken in favor of a deposed king were taken as treason against the one currently in power. As such, accusations of treason were often used as a means for eliminating political enemies or framing opposition.
Sir Thomas Malory was likely familiar with the details of the English Statute of Treasons. Despite not being formally charged with treason, the "knight prisoner" Malory was convicted of a number of egregious crimes, including theft, poaching, extortion, rape, and attempted murder of the Duke of Buckingham. He was also purportedly involved in the War of the Roses, in favor of the house of Lancaster. Edward Hicks writes in his biography of Malory that "though he were involved in the civil strife of Lancaster and York, and dealt with as a traitor by victorious enemies," that Malory not only failed to receive capital punishment, but was permitted to pass his inheritance unto his children. Between 1468 and 1470, four general pardons were granted by Edward IV to incarcerated Lancastrians. Sir Thomas Malory was excluded from all four, and died in prison in 1471, shortly after completing L’Morte d’Arthur. His failure to be outright executed for treason alone, ignoring the various other infractions attributed to him, have led many to believe that Malory may have been a victim of political strong-arming.
Punishments of treason.
French Punishments of treason.
The punishments for those convicted of treason were often brutal and, by modern standards, distastefully contrived, due to the severity of the crime. As an offense against the entire state, traitors were often executed publicly in a manner that would serve as an example, and a warning, for others. Both decapitation and hanging were common execution practices, but a traitor's death "could be clearly distinguished by that of other felons by additional penalties of drawing and/ or quartering." Women accused of treason were often burned at the stake, and clerics were often drowned. Not all punishments of treason were considered "capital." Other, non-capital punishments included the pillory, the wheel, or the removal of the accused's eyes. One source reads that "in 1476, Jean Bon, the would-be assassin of the dauphin Charles, was fortunate not to be punished capitally, though he did have his eyes gouged out." Punishments of treason could also contain a sense of a macabre "poetic justice," or a punishment tailored to fit the crime, as in the instance of counterfeiter Jean Jouye who was boiled alive "presumably because his crime involved the melting down of coin." Imprisonment for traitors was usually the result of a reduced death sentence, rather than of an initial sentencing, and punishments of banishment were announced when the accused escaped. Punishments for petty treason usually involved the paying of fines in the range of 500–18,000 livre tournois for "breach of safeguard, private war, or violation of a truce," and were payable to the plaintiff. Larger fines were enacted for embezzling large sums and were payable to the crown.
English Punishments of Treason.
Medieval English punishments of treason were highly similar to those of the French in terms of both their brutality of their sentencing. Drawing and quartering were common practice to the extent that a quick beheading was considered the privilege of one of noble birth, while a death by hanging, drawing and quartering was reserved for those deemed "ignoble." The English would also disembowel or burn the entrails of those executed for treason, in a practice distinct from the French. Those executed for treason in later medieval England would also often have their bodies put on display to symbolize their shamefulness and their "excision from the body politic." 
Treason in Arthurian Texts.
Treason is a major feature throughout Arthurian romances. Knights constantly commit acts of treason as does Guinevere herself. The treason of Lancelot, Guinevere, Agravain and Mordred eventually destroy Arthur's court and his downfall is caused by the duplicity in his knights. A linking motif throughout a variety of centuries and countries, authors focus on the treasonous destruction of Arthur's court, and the temptation knights face to engage in treason.
Lancelot and Guinevere.
Guinevere.
Queen Guinevere found herself facing accusations of treason several times over the course of recorded Arthurian legend. In Malory's Le Morte d’Arthur, she is accused first in the "Poisoned Apple" episode, when at a dinner she holds for some of the knights of the Round Table, one of the knights, Sir Patrise, dies from eating a poisoned apple intended for Gawain. The real culprit is Sir Pinell, but Guinevere is the accused because it she who gathered the knights for the dinner. Guinevere is accused of treason because, as Malory writes, "the custome was at that tyme that all manner of shameful deth was called treson." The death is defined as treason because "Guinevere's alleged murder of Patryse is a crime against the fellowship, and therefore against Arthur as well." Similarly, in "The Knight of the Cart" episode, Guinevere is accused of treason by the knight Meliagaunt when he discovers blood on her sheets and assumes that she has spent the night with one of her knights who was captured with her. Meligaunt accuses her of being unfaithful to Arthur and even of "hyghe treson." For the French however, the act of adultery would not necessarily have been considered such under the law. In both the case of "the Poisoned Apple" and the "Knight of the Cart," Guinevere is acquitted of the crime through Lancelot dueling to defend her innocence. This trial by combat is used in lieu of an actual refined judicial proceedings, and Lancelot's ability to fight in place of an actual judicial process undermines the legal procedures of the day. It is Lancelot's strength as a knight and not the strength of Arthur's courtly judicial system that ultimately saves Guinevere.
Most famous is the exposing of Guinevere's affair with Lancelot that ultimately leads to the dissolution of the Round Table. Guinevere is accused of treason committed with Lancelot and is sentenced to be burned at the stake for the crime. Her treason consists not only of her adulterous affair with Lancelot, but also because she acted as an accessory to Lancelot's escape and homicide of the knights sent to capture him. English Common law would ordinarily have dictated a trial by one's peers in the case of a treason accusation, and as an accessory Guinevere would not have been able to be sentenced until Lancelot was apprehended. However, Malory cites instead an "Arthurian treason law of his own invention as his basis for the queen's death sentence." Malory writes that in cases where guilt is "manifest and undeniable," that "hasty judgment is appropriate." However, this concept of "hasty judgment" is challenged by Sir Gawain when he protests Arthur's quick sentencing. Because Guinevere can only be considered an accessory to Lancelot's treason, "her treasonous guilt rests in the judgment on the guilt of Lancelot." Malory's depiction of the judicial practice also differs from legal proceedings in his time in that Arthur is able to act as both the plaintiff and the judge. Guinevere's adultery would not even have necessarily been considered treasonous by English common law of the time. Consensual adultery was not defined as treason, but "sexual violation." Robert Kelly writes, "...in western European countries from the twelfth century onwards, adultery and other marital offenses were not criminal, let alone felonious, but under the jurisdiction of ecclesiastical courts." These courts did not issue the death sentence, and though these courts could try her for her part in the affair, only Lancelot's participation could be construed as treasonous for the violation of the king's wife. Instead Guinevere's treasonous crime can only be defined as her status as an accessory to Lancelot's escape and the killing of several knights.
Lancelot.
Lancelot is accused of treason for both his affair with Guinevere and for the homicide of his fellow knights during his escape from the court. Sir Agravain and Sir Mordred initially accuse him of sexual treason for his long-standing affair with Guinevere, which was defined under the law in Malory's time as "if a man violate the king's consort, or the king's eldest unmarried daughter, or the consort of the eldest son and heir of the king." Thus despite the fact that their affair is consensual, Lancelot can be accused of "violating" Guinevere, and committing treason against Arthur by the act.
Lancelot is also accused of treason for the slaying of the knights sent to catch him in his affair. However, it is uncertain whether or not Lancelot could reasonably be accused of treason for the deaths. His guilt lies in whether or not the knights were acting as agents of the king and under his authority. When Agravain and Mordred first raise the charge of adultery against Lancelot, Arthur asks for "proof" of his guilt before he can take action. However, Arthur does not mean proof by trial, but proof by physical evidence. He states that they have his leave to kill Lancelot, only if he is "taken in the dede". Agravain and the knights he gathers to try and catch Lancelot and Guinevere together do not physically catch them in any act of adultery, but rather simply know that they are in Guinevere's bedchamber together. Still, they declare this sufficient evidence to pronounce him traitor, and despite announcing that they will take him to Arthur unharmed, instead try to attack him when he opens the door. Lancelot reacts by arming himself and slaying all the knights who come through the door, including Sir Agravain. Lancelot's slayings would not have been defined as treason under the Statute of 1352, which covered "the slaying of the royal chancellor, treasurer, or justices, but only when they are sitting and acting in their public capacity." Agravain was only given leave to kill Lancelot if his traitorous guilt was made manifest. Thus he was not acting as an agent of the king, but of his own private volition.
Agravain and Mordred.
In Thomas Malory's Morte D’Arthur, Sir Agravain and Sir Mordred set a trap for Sir Lancelot and Guinevere. Gawain warns Agravain not to reveal the affair, fearing the repercussions of Arthur's knowledge of their affair. Agravain ignores Gawain's warning. Malory represents Agravain and Mordred's betrayal of Gawain as the beginning of the downfall of Arthur's court. Their actions are motivated by jealousy for Lancelot's superiority as a knight. In a departure from earlier writings, Malory features Agravain as the main traitor in this passage instead of Mordred. Malory directs the reader to view "Agravain as the agent and Lancelot as the cause for the downfall of the kingdom." Lancelot's adultery is traitorous while simultaneously Agravain and Mordred's reveal of his betrayal are equally, if not more, traitorous. Agravain is killed by Lancelot for his revelation of Lancelot and Guinevere's affair during the escape. Mordred, therefore, becomes the main actor in Arthur's downfall.
Mordred commits the worst act of treason by usurping the throne from Arthur. In the final book of Morte D'Arthur, Gawain explicitly refers to Mordred as a "false traytoure." In the moment when Mordred takes the throne from Arthur, Mordred is "the incarnation of treason." He betrays Arthur as both his knight and his son, committing two acts of treason simultaneously. It is Mordred's evil nature that Malory views as traitorous rather than simply his actions. Mordred is the incarnation of a traitorous knight during the Morte D'Arthur. Malory condemns Mordred's actions and even "invents several passages of treason rhetoric to emphasize the wickedness of such an action." While Agravain is a traitor, Mordred's actions are far worse due to his position as both son and knight of Arthur, and he ought swear fealty to Arthur.
Lanval.
Marie de France authored "Lanval" in the 12th century. "Lanval" is the story of a knight, Lanval, who has a secret fairy lover and rebuffs Guinevere's advances. Guinevere tempts Lanval sexually and he immediately refuses. Her invitation is treason and Lanval's refusal demonstrates his fealty to Arthur. After Lanval refuses, Guinevere accuses Lanval of homosexuality, and homosexuality would have been understood by de France's audience as an act of treason. Sodomy, particularly in homosexual settings, was a charge that mandated burning at the stake in the late Middle Ages. Guinevere's adultery would have brought upon her exactly the same punishment. Sodomy, while a private act, was considered in the late Middle Ages to bring public harm, an act that damaged the whole community. Therefore, sodomy was viewed in the same light as treason during Marie's time. Sodomy accusations in France were so feared that many men openly kept mistresses to ensure no charges were brought against them. Religious figures began aggressively policing sodomy in the 12th century, and many clergymen kept mistresses to rebuff accusations of sodomy. Guinevere's accusation of sodomy is an attempt to ensure her own safety and leverage over Lanval telling anyone about her actions. Both Lanval and Guinevere face the same punishment if their interaction is revealed.
Guinevere's threat attempts to ensnare Lanval into her own treasonous acts, threatening to sic a legal system on him that would demand his burning should he engage in sodomy. While Lanval does not commit any treasonous acts, Guinevere has the power to accuse him of treasonous acts should she so desire. Particularly in Marie's time, readers would have been aware of the power an accusation of sodomy had. If Lanval had been convicted of sodomy he would be burned at the stake.
Gawain.
Sir Gawain is bloodthirsty and often murderous. His own brother Gareth recognizes his tempestuous relationship with those he does not like, and warns his brother not to act rashly when he is unhappy. These elements of Gawain's character play an important part in the last book of Thomas Malory's Morte D'Arthur. Gawain urges Arthur to avenge his brothers' deaths and "pushes Arthur to pursue Lancelot to France and leave Mordred in charge of England." Mordred's rule of England brings about the downfall of Arthur's kingdom and Gawain's vengeful spirit facilitates the end of Arthur's rule. Gawain's actions enable the treasonous acts of Mordred. Gawain is simultaneously a traitor in his unwillingness to reveal the affair to Arthur and his concealment is a violation of Arthur's trust.
Gawain and the Green Knight.
Gawain is duplicitous throughout Sir Gawain and the Green Knight. He is tempted by the wife of Bertilak, a lord who offers his home to Gawain. While Gawain eventually learns the harms of duplicity, Gawain violates "his responsibility to honor the host–guest relationship". Gawain keeps the magical girdle, refusing to honor his agreement with Bertilak that he will give everything Lady Bertilak gives to Gawain to the Lord. Instead, Gawain hides the girdle. This treason against the Lord necessitates a physical punishment from the Green Knight before "Gawain can return to his proper place in Arthurian society". Treason is committed by Gawain against the Lord and therefore he must be physically punished, as one would be during the Middle Ages for crimes of treason. In this case, however, Gawain's punishment is not death but simply a nick on his neck. This permits Gawain to be punished, but not killed, while still demonstrating the importance of honesty. Gawain's instinct is to act duplicitously but he is chastised for these instances.