its wrong what he did afterwards I will agree...but shooting him on the floor was his decision and he alone should only be able to judge what he did was right or not...
That's not how the system works, and we should all be thankful for that fact.
Do we REALLY want people to be able to do whatever they think is right and then let them be the final authority in terms of judging whether what they did was right or not? I certainly don't.
When someone is murdered, the murderer is the WORST possible person to adjudicate the rightness/wrongness of his own actions.