How to understand a second order Karma? 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.
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 &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.
  
  "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.
  
  "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.
  
  "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.


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 &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."


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.