Well, as with most things, common sense must be applied to these situations, both in how they are handled in real time, and how we see them afterwards.

For me, if I genuinely thought my life was in danger, or one of my children, I was not hesitate to act and eliminate that threat. Does this mean I would go home and sleep fine? Probably not. I am human, and I am a pretty empathetic person, so I can only assume I would be pretty fucked up from the experience. But again, if I truly feared for my life or the life of a loved one, I would do it, knowing full well the psychological effect it would have on me.

Someone who would do this and feel no remorse, or sleep just fine, is obviously someone that enjoyed the act, or at the very least values human life less than they should, or their moral compass has been skewed into the danger zone.

It's about perspective, and taking an objective look at the situation and circumstances.