I think it has a lot to do with how society responds to the killing. If he is universally hailed as a hero, he is much less likely to have problems with it at any time. If the leftists keep telling him how he must be suffering inside for having done that, then that's what he will eventually start to feel. The human species is very much psychologically influenced by societal reactions to their conduct. It's almost unavoidable. But, in my opinion, the appropriate response for the boy to feel is pride in having done the right and courageous thing when the moment of truth was upon him. This is the response we should, as a society, encourage in him.