For me, I couldn't pick up a weapon against another person. I just think it's wrong. Most of my morals are pretty slack, and I can be pretty darn aggressive, but picking up weapons and aiming them at other people just isn't something I can imagine doing in anything other than a blind rage- by which time you've pretty much lost anyway.
Anger leads to the dark side, and all that.
Where is this hypothetical situation where my children are all only saveable by use of weapons? I haven't actually come across that scenario in my 23 years of parenthood. It's fairly likely I'd do myself greater damage than I could inflict on anyone else, what with complete ineptitude and inexperience with, you know, death battles.
Thing is with the sniper, he signed up to the army. He already knew how to shoot things and chose to surround himself with a 'family' who were being fired at. You don't chose your family, isn't that the adage? But these are people and a situation he chose, and then self justified his actions by describing his motivation as saving his family. How handy.