To answer this question, you really have to be put in the situation. Even if I
thought my girls were in trouble, I couldn't kill the person responsible until I saw it in progress - that's not to say I wouldn't want to. This guy was threatening my father with all sorts of legal shit once, I really felt like I wanted to kill him, I confronted him with a hammer..........I didn't do a thing, again, not that I didn't want to.
Even in the army, I couldn't bring myself to shoot until we were being shot at.