Wooot for subjective ethics!!! In my branch of ethics choice exists as a group but not as an individual
So I believe that the only real solution to this problem is counselling, to help give him enough influence to not perform the action of murder next time around, and also to help him deal with the fact that he committed murder. Remember, two wrongs don't make a right (three rights make a left though) so taking away years of his life doesn't make things right, only makes the victims feel better (it's like if someone was killed because they imprisoned someone with adequate food and clothing and stuff for 25 years (only this is in reverse order)).
Summary: Counselling is what is right IMO as I think it isn't right to imprison someone for an act against someone who wronged them (if it is severe enough) as that just puts more morally wrong actions in the mix rather than fixing the problem.
Basically, as a group, different people will do different things in different situations, but individually, you would always do the action which you did unless the influences to that event were different (i.e. better human dvelopment, different social upbringin, or maybe just some random memory which prevents an action (in which case I believe it to be a completely different scenario as the variable (in this case mental influences) are not the same))
Summary: Counselling is what is right IMO as I think it isn't right to imprison someone for an act against someone who wronged them (if it is severe enough) as that just puts more morally wrong actions in the mix rather than fixing the problem.