The amount of pent up rage I have inside me I could quite easily kill someone if I knew there were no consequences to my actions. Of course it would be people whom I felt deserved it.
I wouldn't just "kill" them of course, I'd make them fight, if they kill me instead then that's fair, I see no point in taking someone out with a gun.
Of course unless the world descends into total anarchy and all social order and rules no longer apply, most of us will never be in a situation where we could really say yes or no, anything we say now is mere speculation. I'm sure there are some who have said no they could never kill someone, when faced with the kill or be killed scenario, would rather take the others life.