Although the subject matter of Poe's story is a murder, "The Cask of Amontillado" is not a tale of detection like "The Murders in the Rue Morgue" or "The Purloined Letter"; there is no investigation of Montresor's crime and the criminal himself explains how he committed the murder.
The mystery in "The Cask of Amontillado" is in Montresor's motive for murder.
Without a detective in the story, it is up to the reader to solve the mystery.
From the beginning of the story, it is made clear that Montresor has exaggerated his grievances towards Fortunato.
The reader is led to assume that much like his exaggerated grievances, the punishment he chooses will represent what he believes is equal justice, and in turn, going to the extreme.

Montresor never specifies his motive beyond the vague "thousand injuries" and "when he ventured upon insult" to which he refers.
Some context is provided, including Montresor's observation that his family once was great (but no longer so), and Fortunato's belittling remarks about Montresor's exclusion from Freemasonry.
Many commentators conclude that, lacking significant reason, Montresor must be insane, though even this is questionable because of the intricate details of the plot.

There is also evidence that Montresor is almost as clueless about his motive for revenge as his victim.
In his recounting of the murder, Montresor notes, "A wrong is unredressed when retribution overtakes its redresser.
It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong".
After Fortunato is chained to the wall and nearly entombed alive, Montresor merely mocks and mimics him, rather than disclosing to Fortunato the reasons behind his exacting revenge.
Montresor may not have been entirely certain of the exact nature of the insults for which he expected Fortunato to atone.

Additional scrutiny into the vague injuries and insults may have to do with a simple matter of Montresor's pride and not any specific words from Fortunato.
Montresor comes from an established family.
His house had once been noble and respected, but has fallen slightly in status.
Fortunato, as his name would seem to indicate, has been blessed with good fortune and wealth and is, therefore, viewed as unrefined by Montresor; however, this lack of refinement has not stopped Fortunato from surpassing Montresor in society, which could very well be the "insult" motive for Montresor's revenge.

There is indication that Montresor blames his unhappiness and loss of respect and dignity within society on Fortunato.
It is easy to ascertain that Fortunato is a Freemason, while Montresor is not, which could be the source of Fortunato's recent ascension into upper class society.
Montresor even imparts this blame to Fortunato when he states, "You are rich, respected, admired, beloved; you are happy, as once I was".
This interchanging of fortunes is a suggestion that, since the names Montresor and Fortunato mirror one another, there is a psychological reciprocal identification between victim and executioner.
This identification reciprocity is further suggested when one takes into consideration that Montresor entombs Fortunato in the Montresor family catacombs rather than dispatching him elsewhere in the city amidst the chaos of the Carnival.
It is with this converging of the two characters that one is able to see the larger symbolism of the Montresor crest – the foot steps on the serpent while the serpent forever has his fangs embedded in the heel.

Upon further investigation into the true nature of character, double meaning can be derived from the Montresor crest.
It is the position of Montresor to view himself as the owner of the righteous foot that is crushing the insolent Fortunato serpent and his "thousand injuries" that progress into insult.
A more allegoric meaning of Poe's places the actors in reverse.
The blind oaf Fortunato has unintentionally stepped upon the snake in the grass – the sneaky and cunning Montresor – who, as a reward for this accidental bruising, sinks his fangs deep into the heel of his offend
Although the subject matter of Poe's story is a murder, "The Cask of Amontillado" is not a tale of detection like "The Murders in the Rue Morgue" or "The Purloined Letter"; there is no investigation of Montresor's crime and the criminal himself explains how he committed the murder. The mystery in "The Cask of Amontillado" is in Montresor's motive for murder. Without a detective in the story, it is up to the reader to solve the mystery. From the beginning of the story, it is made clear that Montresor has exaggerated his grievances towards Fortunato. The reader is led to assume that much like his exaggerated grievances, the punishment he chooses will represent what he believes is equal justice, and in turn, going to the extreme.
Montresor never specifies his motive beyond the vague "thousand injuries" and "when he ventured upon insult" to which he refers. Some context is provided, including Montresor's observation that his family once was great (but no longer so), and Fortunato's belittling remarks about Montresor's exclusion from Freemasonry. Many commentators conclude that, lacking significant reason, Montresor must be insane, though even this is questionable because of the intricate details of the plot.
There is also evidence that Montresor is almost as clueless about his motive for revenge as his victim. In his recounting of the murder, Montresor notes, "A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong". After Fortunato is chained to the wall and nearly entombed alive, Montresor merely mocks and mimics him, rather than disclosing to Fortunato the reasons behind his exacting revenge. Montresor may not have been entirely certain of the exact nature of the insults for which he expected Fortunato to atone.
Additional scrutiny into the vague injuries and insults may have to do with a simple matter of Montresor's pride and not any specific words from Fortunato. Montresor comes from an established family. His house had once been noble and respected, but has fallen slightly in status. Fortunato, as his name would seem to indicate, has been blessed with good fortune and wealth and is, therefore, viewed as unrefined by Montresor; however, this lack of refinement has not stopped Fortunato from surpassing Montresor in society, which could very well be the "insult" motive for Montresor's revenge.
There is indication that Montresor blames his unhappiness and loss of respect and dignity within society on Fortunato. It is easy to ascertain that Fortunato is a Freemason, while Montresor is not, which could be the source of Fortunato's recent ascension into upper class society. Montresor even imparts this blame to Fortunato when he states, "You are rich, respected, admired, beloved; you are happy, as once I was". This interchanging of fortunes is a suggestion that, since the names Montresor and Fortunato mirror one another, there is a psychological reciprocal identification between victim and executioner. This identification reciprocity is further suggested when one takes into consideration that Montresor entombs Fortunato in the Montresor family catacombs rather than dispatching him elsewhere in the city amidst the chaos of the Carnival. It is with this converging of the two characters that one is able to see the larger symbolism of the Montresor crest – the foot steps on the serpent while the serpent forever has his fangs embedded in the heel.
Upon further investigation into the true nature of character, double meaning can be derived from the Montresor crest. It is the position of Montresor to view himself as the owner of the righteous foot that is crushing the insolent Fortunato serpent and his "thousand injuries" that progress into insult. A more allegoric meaning of Poe's places the actors in reverse. The blind oaf Fortunato has unintentionally stepped upon the snake in the grass – the sneaky and cunning Montresor – who, as a reward for this accidental bruising, sinks his fangs deep into the heel of his offend
The Cask of Amontillado