user-1: If a person B does an intentional harmful action on person A, and so B does a bad karma.
As a result of that karma, it happens that a person C ends up hurting person B, intentionally or unintentionally. 
So what is the result of the action of person C? 
Does her action account for Karma?
Does she even have any choice to hurt B or not hurt B? 
If C hurts B intentionally does that account for bad karma, but C did in because she had to as it was B's karma?
If this continues, this whole sequence will never ever end. 

user111: 
O! Bhikkhu monks, it is volition - cetana that I call Kamma,
  -cetanaham bhikkhave kammam vadami

If A, B or C has an unwholesome thought or speech/action caused by an unwholesome thought, it is bad Karma. It's that simple.
An intention to harm a person is always unwholesome whether that person deserves it or not.

user12123: 
If this continues, this whole sequence will never ever end.

That is why the sanasara is endless loop till we understand it and break it. 
B hurt A intentionaly and for that karma B get hurt by C intentionally or not, if C didn't someone else will, till the end of B bad karma. Another thing only the Lord Buddha is able to understand the way of karma works, all behind the scence are only understandable to Buddhas only.

user-1: 
As a result of that karma, it happens that a person C ends up hurting person B, intentionally or unintentionally.
So what is the result of the action of person C?

If the action is done intentionally, the result for person C is future unwholesome resultants.
If the action is done unintentionally, the result for person C is no unwholesome kamma. See the Twin verses in Dhp. v1-2.

Does she even have any choice to hurt B or not hurt B?

We always have a choice whether or not to act on an intention. Person C does not have to act on the intention to harm person B. She can choose to observe that intention with mindfulness and instead cultivate insights from it.

If C hurts B intentionally does that account for bad karma, but C did in because she had to as it was B's karma?

Every action that originates in the Three Unwholesome Roots (greed, hatred and delusion) will result in future unwholesome resultants.

If this continues, this whole sequence will never ever end.

It can end. It will end if one works out ones own liberation. The Buddha did it. Many of his disciples and followers did it too. Every person has within himself or herself, the potentiality to become free from suffering, to achieve Nibbana, to become a Buddha.

user471: In Buddhism, kamma and its fruits is not a system of universal justice. It simply is cause and effect. An individual's intention is the most important thing in kamma.
If a person has evil intentions and commits evil acts, then he may end up in unhappy destinations. Even if he comes back as a human being, he may be sickly or short-lived or ugly or poor etc. Please read the rest of MN 135 for details. If based on his intentions, he harmed others, then in future, he becomes sickly. If based on his intentions, he was miserly and did not donate to others, then he may become poor in future.
According to MN 135:

The Blessed One said: "There is the case, student, where a woman or
  man is a killer of living beings, brutal, bloody-handed, given to
  killing & slaying, showing no mercy to living beings. Through having
  adopted & carried out such actions, on the break-up of the body, after
  death, he/she reappears in the plane of deprivation, the bad
  destination, the lower realms, hell. If, on the break-up of the body,
  after death — instead of reappearing in the plane of deprivation, the
  bad destination, the lower realms, hell — he/she comes to the human
  state, then he/she is short-lived wherever reborn. This is the way
  leading to a short life: to be a killer of living beings, brutal,
  bloody-handed, given to killing & slaying, showing no mercy to living
  beings.
"But then there is the case where a woman or man, having abandoned the
  killing of living beings, abstains from killing living beings, and
  dwells with the rod laid down, the knife laid down, scrupulous,
  merciful, & sympathetic for the welfare of all living beings. Through
  having adopted & carried out such actions, on the break-up of the
  body, after death, he/she reappears in a good destination, in the
  heavenly world. If, on the break-up of the body, after death — instead
  of reappearing in a good destination, in the heavenly world — he/she
  comes to the human state, then he/she is long-lived wherever reborn.
  This is the way leading to a long life: to have abandoned the killing
  of living beings, to abstain from killing living beings, to dwell with
  one's rod laid down, one's knife laid down, scrupulous, merciful, &
  sympathetic for the welfare of all living beings.
"There is the case where a woman or man is one who harms beings with
  his/her fists, with clods, with sticks, or with knives. Through having
  adopted & carried out such actions, on the break-up of the body, after
  death, he/she reappears in the plane of deprivation... If instead
  he/she comes to the human state, then he/she is sickly wherever
  reborn. This is the way leading to sickliness: to be one who harms
  beings with one's fists, with clods, with sticks, or with knives.
"But then there is the case where a woman or man is not one who harms
  beings with his/her fists, with clods, with sticks, or with knives.
  Through having adopted & carried out such actions, on the break-up of
  the body, after death, he/she reappears in a good destination... If
  instead he/she comes to the human state, then he/she is healthy
  wherever reborn. This is the way leading to health: not to be one who
  harms beings with one's fists, with clods, with sticks, or with
  knives.

However, not everything we experience is caused by kamma according to SN36.21:

"There are cases where some feelings arise based on bile... phlegm...
  based on internal winds... based on a combination of bodily humors...
  from the change of the seasons... from uneven care of the body... from
  harsh treatment... from the result of kamma. You yourself should know
  how some feelings arise from the result of kamma. Even the world is
  agreed on how some feelings arise from the result of kamma. So any
  brahmans & contemplatives who are of the doctrine & view that whatever
  an individual feels — pleasure, pain, neither pleasure-nor-pain — is
  entirely caused by what was done before — slip past what they
  themselves know, slip past what is agreed on by the world. Therefore I
  say that those brahmans & contemplatives are wrong."

So it is pointless to assume that if somebody hit you, it must be caused by you hitting someone else in the past. It may not have any kamma-related cause. In the quote above, "harsh treatment" is separated from "result of kamma". So, they are separate causes. "Uneven care of the body" and "change of seasons" are also other separate causes. So, if somebody hit you and it caused pain, then it may well be just "harsh treatment" from someone else.
How to alleviate one's karma is explained in the Lonaphala Sutta or the Discourse of the Salt Crystal.

user254: So what is the result of the action of person C?
I think it would be unusual, a supernatural ability (e.g. of the Buddha's), to be able to see/know/explain the result of kamma.
I think it's called,

"Divine eye" (dibba-cakkhu), that is, knowing others' karmic destinations

For this reason (i.e. that few people have this supernatural ability), all questions of the form "what's the precise karmic result of such-and-such action" asked on this site are pointless or unanswerable.
People can explain a little more about the doctrine of karma-in-general (e.g. here), but I think we can only give some fairly general statements from the suttas, for example that "karma is intention".
Does she even have any choice to hurt B or not hurt B?
I think people debate (or question) whether or to what extent "free will" exists -- whether people are able to choose or whether choices are predetermined -- see e.g. Predetermined future vs. Free will in Buddhism
I think the doctrine says though that if you are person C then you are heir to your own kamma.
If this continues, this whole sequence will never ever end.
I think the doctrine allows for kammic seeds which never come to fruition (which is how it can end):

Kamma and its Fruit (by Nyanaponika Thera)
Most writings on the doctrine of kamma emphasize the strict lawfulness governing kammic action, ensuring a close correspondence between our deeds and their fruits. While this emphasis is perfectly in place, there is another side to the working of kamma — a side rarely noted, but highly important. This is the modifiability of kamma, the fact that the lawfulness which governs kamma does not operate with mechanical rigidity but allows for a considerably wide range of modifications in the ripening of the fruit.
If kammic action were always to bear fruits of invariably the same magnitude, and if modification or annulment of kamma-result were excluded, liberation from the samsaric cycle of suffering would be impossible; for an inexhaustible past would ever throw up new obstructive results of unwholesome kamma.

Also I think that the doctrine tells us to the kammic principle intentionally and skilfully, e.g.:

Intending good for good to ripen (and/or to make any small evil relatively negligible, like a lump of salt dissolved in a river)
Intending cessation for cessation to ripen (I think, more specifically -- abandoning greed, ignorance, and aversion; abandoning self-view; and the noble eightfold path).


-1: <p>If a person <strong>B</strong> does an intentional harmful action on person <strong>A</strong>, and so <strong>B</strong> does a bad karma.</p>

<p>As a result of that karma, it happens that a person <strong>C</strong> ends up hurting person <strong>B</strong>, intentionally or unintentionally. </p>

<p>So what is the result of the action of person <strong>C</strong>? </p>

<p>Does her action account for Karma?</p>

<p>Does she even have any choice to hurt B or not hurt B? </p>

<p>If <strong>C</strong> hurts <strong>B</strong> intentionally does that account for bad karma, but <strong>C</strong> did in because she had to as it was <strong>B</strong>'s karma?</p>

<p>If this continues, this whole sequence will never ever end. </p>
 111: <blockquote>
  <p>O! Bhikkhu monks, it is volition - cetana that I call Kamma,
  -cetanaham bhikkhave kammam vadami</p>
</blockquote>

<p>If A, B or C has an unwholesome thought or speech/action caused by an unwholesome thought, it is bad Karma. It's that simple.</p>

<p>An intention to harm a person is always unwholesome whether that person deserves it or not.</p>
 12123: <blockquote>
  <p>If this continues, this whole sequence will never ever end.</p>
</blockquote>

<p>That is why the sanasara is endless loop till we understand it and break it. 
<strong>B</strong> hurt <strong>A</strong> intentionaly and for that karma <strong>B</strong> get hurt by <strong>C</strong> intentionally or not, if <strong>C</strong> didn't someone else will, till the end of <strong>B</strong> bad karma. Another thing only the Lord Buddha is able to understand the way of karma works, all behind the scence are only understandable to Buddhas only.</p>
 -1: <blockquote>
<p>As a result of that karma, it happens that a person C ends up hurting person B, intentionally or unintentionally.</p>
<p>So what is the result of the action of person C?</p>
</blockquote>
<p>If the action is done intentionally, the result for person C is future unwholesome resultants.</p>
<p>If the action is done unintentionally, the result for person C is no unwholesome kamma. See the <a href="http://buddhanet.net/dhammapada/d_twin.htm" rel="nofollow noreferrer"><em>Twin verses in Dhp. v1-2</em></a>.</p>
<blockquote>
<p>Does she even have any choice to hurt B or not hurt B?</p>
</blockquote>
<p>We always have a choice whether or not to act on an <em>intention</em>. Person C does not have to act on the intention to harm person B. She can choose to observe that intention with mindfulness and instead cultivate insights from it.</p>
<blockquote>
<p>If C hurts B intentionally does that account for bad karma, but C did in because she had to as it was B's karma?</p>
</blockquote>
<p>Every action that originates in the <a href="https://en.wikipedia.org/wiki/Three_poisons#Sanskrit/Pali/Tibetan_terms_and_translations" rel="nofollow noreferrer"><em>Three Unwholesome Roots (greed, hatred and delusion)</em></a> will result in future unwholesome resultants.</p>
<blockquote>
<p>If this continues, this whole sequence will never ever end.</p>
</blockquote>
<p>It can end. It will end if one works out ones own liberation. The Buddha did it. Many of his disciples and followers did it too. Every person has within himself or herself, the potentiality to become free from suffering, to achieve Nibbana, to become a Buddha.</p>
 471: <p>In Buddhism, kamma and its fruits is not a system of universal justice. It simply is cause and effect. An individual's intention is the most important thing in kamma.</p>

<p>If a person has evil intentions and commits evil acts, then he may end up in unhappy destinations. Even if he comes back as a human being, he may be sickly or short-lived or ugly or poor etc. Please read the rest of MN 135 for details. If based on his intentions, he harmed others, then in future, he becomes sickly. If based on his intentions, he was miserly and did not donate to others, then he may become poor in future.</p>

<p>According to <a href="https://www.accesstoinsight.org/tipitaka/mn/mn.135.than.html" rel="nofollow noreferrer">MN 135</a>:</p>

<blockquote>
  <p>The Blessed One said: "There is the case, student, where a woman or
  man is a killer of living beings, brutal, bloody-handed, given to
  killing &amp; slaying, showing no mercy to living beings. Through having
  adopted &amp; carried out such actions, on the break-up of the body, after
  death, he/she reappears in the plane of deprivation, the bad
  destination, the lower realms, hell. If, on the break-up of the body,
  after death — instead of reappearing in the plane of deprivation, the
  bad destination, the lower realms, hell — he/she comes to the human
  state, then he/she is short-lived wherever reborn. This is the way
  leading to a short life: to be a killer of living beings, brutal,
  bloody-handed, given to killing &amp; slaying, showing no mercy to living
  beings.</p>
  
  <p>"But then there is the case where a woman or man, having abandoned the
  killing of living beings, abstains from killing living beings, and
  dwells with the rod laid down, the knife laid down, scrupulous,
  merciful, &amp; sympathetic for the welfare of all living beings. Through
  having adopted &amp; carried out such actions, on the break-up of the
  body, after death, he/she reappears in a good destination, in the
  heavenly world. If, on the break-up of the body, after death — instead
  of reappearing in a good destination, in the heavenly world — he/she
  comes to the human state, then he/she is long-lived wherever reborn.
  This is the way leading to a long life: to have abandoned the killing
  of living beings, to abstain from killing living beings, to dwell with
  one's rod laid down, one's knife laid down, scrupulous, merciful, &amp;
  sympathetic for the welfare of all living beings.</p>
  
  <p>"There is the case where a woman or man is one who harms beings with
  his/her fists, with clods, with sticks, or with knives. Through having
  adopted &amp; carried out such actions, on the break-up of the body, after
  death, he/she reappears in the plane of deprivation... If instead
  he/she comes to the human state, then he/she is sickly wherever
  reborn. This is the way leading to sickliness: to be one who harms
  beings with one's fists, with clods, with sticks, or with knives.</p>
  
  <p>"But then there is the case where a woman or man is not one who harms
  beings with his/her fists, with clods, with sticks, or with knives.
  Through having adopted &amp; carried out such actions, on the break-up of
  the body, after death, he/she reappears in a good destination... If
  instead he/she comes to the human state, then he/she is healthy
  wherever reborn. This is the way leading to health: not to be one who
  harms beings with one's fists, with clods, with sticks, or with
  knives.</p>
</blockquote>

<p>However, not everything we experience is caused by kamma according to <a href="https://www.accesstoinsight.org/tipitaka/sn/sn36/sn36.021.than.html" rel="nofollow noreferrer">SN36.21</a>:</p>

<blockquote>
  <p>"There are cases where some feelings arise based on bile... phlegm...
  based on internal winds... based on a combination of bodily humors...
  from the change of the seasons... from uneven care of the body... from
  harsh treatment... from the result of kamma. You yourself should know
  how some feelings arise from the result of kamma. Even the world is
  agreed on how some feelings arise from the result of kamma. So any
  brahmans &amp; contemplatives who are of the doctrine &amp; view that whatever
  an individual feels — pleasure, pain, neither pleasure-nor-pain — is
  entirely caused by what was done before — slip past what they
  themselves know, slip past what is agreed on by the world. Therefore I
  say that those brahmans &amp; contemplatives are wrong."</p>
</blockquote>

<p>So it is pointless to assume that if somebody hit you, it must be caused by you hitting someone else in the past. It may not have any kamma-related cause. In the quote above, "harsh treatment" is separated from "result of kamma". So, they are separate causes. "Uneven care of the body" and "change of seasons" are also other separate causes. So, if somebody hit you and it caused pain, then it may well be just "harsh treatment" from someone else.</p>

<p>How to alleviate one's karma is explained in the <a href="https://www.accesstoinsight.org/tipitaka/an/an03/an03.099.than.html" rel="nofollow noreferrer">Lonaphala Sutta</a> or the Discourse of the Salt Crystal.</p>
 254: <p><strong>So what is the result of the action of person C?</strong></p>
<p>I think it would be unusual, a supernatural ability (e.g. of the Buddha's), to be able to see/know/explain the result of kamma.</p>
<p>I think <a href="https://en.wikipedia.org/wiki/Abhij%C3%B1%C4%81#Enumerations_of_special_knowledges" rel="nofollow noreferrer">it's called</a>,</p>
<blockquote>
<p>&quot;Divine eye&quot; (dibba-cakkhu), that is, knowing others' karmic destinations</p>
</blockquote>
<p>For this reason (i.e. that few people have this supernatural ability), all questions of the form &quot;what's the precise karmic result of such-and-such action&quot; asked on this site are pointless or unanswerable.</p>
<p>People can explain a little more about the doctrine of karma-in-general (<a href="https://buddhism.stackexchange.com/a/23830/254">e.g. here</a>), but I think we can only give some fairly general statements from the suttas, <a href="https://accesstoinsight.org/search_results.html?q=kamma&amp;sa=Search" rel="nofollow noreferrer">for example</a> that &quot;karma is intention&quot;.</p>
<p><strong>Does she even have any choice to hurt B or not hurt B?</strong></p>
<p>I think people debate (or question) whether or to what extent &quot;free will&quot; exists -- whether people are able to choose or whether choices are predetermined -- see e.g. <a href="https://buddhism.stackexchange.com/q/2762/254">Predetermined future vs. Free will in Buddhism</a></p>
<p>I think the doctrine says though that if you are person C then you are heir to your own kamma.</p>
<p><strong>If this continues, this whole sequence will never ever end.</strong></p>
<p>I think the doctrine allows for kammic seeds which never come to fruition (which is how it can end):</p>
<blockquote>
<p><a href="https://www.accesstoinsight.org/lib/authors/nyanaponika/kammafruit.html" rel="nofollow noreferrer">Kamma and its Fruit</a> (by Nyanaponika Thera)</p>
<p>Most writings on the doctrine of kamma emphasize the strict lawfulness governing kammic action, ensuring a close correspondence between our deeds and their fruits. While this emphasis is perfectly in place, there is another side to the working of kamma — a side rarely noted, but highly important. This is the modifiability of kamma, the fact that the lawfulness which governs kamma does not operate with mechanical rigidity but allows for a considerably wide range of modifications in the ripening of the fruit.</p>
<p>If kammic action were always to bear fruits of invariably the same magnitude, and if modification or annulment of kamma-result were excluded, liberation from the samsaric cycle of suffering would be impossible; for an inexhaustible past would ever throw up new obstructive results of unwholesome kamma.</p>
</blockquote>
<p>Also I think that the doctrine tells us to the kammic principle intentionally and skilfully, e.g.:</p>
<ul>
<li>Intending good for good to ripen (and/or to make any small evil relatively negligible, like a lump of salt dissolved in a river)</li>
<li>Intending cessation for cessation to ripen (I think, more specifically -- abandoning greed, ignorance, and aversion; abandoning self-view; and the noble eightfold path).</li>
</ul>
