How to understand a second order Karma?

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.

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.

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.

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.