Are you serious? You could not in any way justify killing another human being in defense of anyone other than yourself? Really?
No, not what I sad (or meant). I would with no problem kill someone to protect someone else or me. I would have a problem killing someone that posed no problem.
Either way, I would have to live with that for the rest of my life and it would bother be greatly! I imagine that I would become mentally unstable due to killing another human being...but if justified, protecting others, not a problem.. If it was to protect myself, I would probably have nightmares about it...