For what reason did he commit the killing?
Options:
- investigation
- disease
- punishment
- retribution
- going to jail
Let's think now! Step-by-step reasoning:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Retribution means punishment for a crime. Retribution is what judge gives after studying the case or after a criminal trial. Retribution was the reason for why he commits the killing.
So, the final answer is retribution.