Killing someone is an act that is accomplished by many thought processes. A single thought process can have a maximum of 17 cittas. Among them, the cittas 9 to 15 are called the Javana cittas. They are the ones responsible for creating Karma.
1. The subject, the consciousness, receives objects from within and without. When a person is in a state of profound sleep his mind is
said to be vacant, or, in other words, in a state of bhavanga. We
always experience such a passive state when our minds do not respond
to external objects. This flow of bhavanga is interrupted when objects
enter the mind.
2. Then the bhavanga consciousness vibrates for one
thought-moment and passes away.
3. A bhavanga that cuts off the flow (bhavanga upaccheda).
4. Thereupon the sense-door consciousness (pañca-dvārāvajjana)
arises and ceases. At this stage the natural flow is checked and is
turned towards the object.
5. Immediately after there arises and ceases the eye consciousness* (cakkhu viññāna), but yet knows no more about it.
6. This sense operation is followed by a moment of reception of the object so seen (sampaticchana).
7. Next comes the investigating faculty (santīrana) or a momentary examination of the object so received.
8. After this comes that stage of representative cognition termed the determining consciousness (votthapana). Discrimination is
exercised at this stage. Free Will plays its part here.
9 to 15. Immediately after there arises the psychologically most important stage - Impulsion or javana. It is at this stage that an
action is judged whether moral or immoral. Kamma is performed at this
stage; if viewed rightly (yoniso manasikāra), the javana becomes
moral; if viewed wrongly (ayoniso manasikāra), it becomes immoral. In
the case of an Arahat this javana is neither moral nor immoral, but
merely functional (kiriya). This javana stage usually lasts for seven
thought moments, or, at times of death, five.
16 to 17. The whole process which happens in an infinitesimal part of time ends with the registering consciousness (tadālambana), lasting
for two thought-moments - thus completing one thought-process at the
expiration of seventeen thought-moments.
Each of the Javana Cittas are Karmically potent. They come into fruitions at different times.
1. Immediately effective kamma (dittha-dhamma-vedaniya kamma)
- kamma which produces its results in the present life.
Among the seven javana cittas, the first, or the weakest javana-citta called the dittha-dhamma-vedaniya - produces its effects in the present life. Because it's the weakest kamma, it is unable to produce results past the current life.
If this kamma is performed in this present life and does meet the opportunity to ripen , it will give the result in the same existence. Otherwise, the effect becomes defunct.
2. Subsequently effective kamma (upapajja-vedaniya kamma )
- kamma which produces its effects in the nearest next life.
Among the seven javana process, this is the seventh one.The second weakest of the javana cittas, - called upapajja-vedaniya kamma.
3. indefinitely effective kamma ( aparā-pariya-vedaniya kamma)
- kamma which produces its effects in lives subsequent to nearest next life.
It can ripen at any time whenever it has opportunity to produce the results from the second future existence onwards as long as the round of rebirths continues. It never becomes defunct until it comes to fruition.
4. defunct kamma (ahosi kamma)
- kamma which is ineffective.
The kamma that is unable to produce its results are called ahosi-kamma.
If immediately effective kamma (dittha-dhamma-vedaniya kamma), for example, is unable to operate in this life, and if the subsequently effective kamma (upapajja-vedaniya kamma) does not operated in the second birth or next life, they are ahosi-kamma.
In the case of Arahants, all their accumulated good or bad kamma from the past which was due to ripen in the future lives become defunct with their enlightenment.
Since killing a person involves so many thought processes, it doesn't end just because the Karma of one of the Javana cittas of one thought process comes into fruition. Chances are that one is going to pay for the Javana cittas in the multitudes of thought processes involved with the killing, from time to time in Samsara. So getting killed once in a future life is not likely to complete all results of the Karmas one accumulates in an act of killing.
If person B kills person A in next life, it won't be a new karma for person A as the fruit is never a Karma. However it will become a new Karma for person B. Because B is not the one facing the result. He's a different person creating a fresh Karma.