i could very, very, VERY easily see myself beating the buh-geezus outta someone, had they threatened or hurt family or a friend. without thought, without hesitation, without explanation -- bare hands, baseball bat, car... wouldnt matter.
in my mind, tho, there's a HUGE split between that and death (all be it relatively slight, on a legal basis).
in a personal revenge type case, like if a girl cheated on me... i think i'd be sorely tempted to break'em both. again, not kill, but probably pretty close. if thats an insecurity, so be it.
putting myself in the situation of a cop and/or a soldier... i dunno what i'd do. i often want to become one of the two, sorta like a real-life superhero. but would i be able to kill, even if i needed to? ...i honestly dont know. im not even sure how comfortable id feel shooting at someone (not to kill).
bottom line -- could i kill someone? its possible, but ill tell you now, i cant think of a single situation where i'd need to.