There's 2 people standing in a place where nobody will find them or hear them, one is on the ground and the other has a gun to the first guy's head about to kill him. Assuming we could boil the consequences down to only yourself, no police or anything, would you sacrifice yourself or be the guy that kills him?
Basically, you choose if you want to be the guy about to murder the other and live with the thought of killing a man your whole life or the guy about to get murdered.