I couldn't answer that question until I was forced into the situation. I could sit here right now and say "No. That's a terrible thing to do!", but you never know. I was in the Army during the little pissing contest in Iraq, and considered it then, but never really came up with a pat answer.
I really think if it came down to it, I probably could if I had to. Not out of malice or forethought, but as a reaction to the safety of myself or others. I wouldn't go back two weeks after I found a guy was sleeping with my wife and kill him or her, but if her life were in mortal danger and it was him or her, I probably wouldn't have to think twice about it. Guess it's just the way I've been raised...
I've spent nearly half my life in either military or para-military (law enforcement) situations where people dying is just a fact of life...
Not to sound cold or uncaring, but when you've been around some of that stuff for as long as I have, you tend to distance yourself emotionally from what's going on. You always hear about how cops and people in similar situations have very morbid senses of humor to help them cope with what's going on around them, and it's very true. You and I could hear the same story on the news about something terrible happening to somebody and our reactions would probably be polar opposites from each other. You might be revulsed or feel pity for them, where as I would probably started telling sick jokes. It's disgusting, but it's the way we are...