If they sneak up on you, then you're probably dead anyways. Taking cover and firing, like I said, gives away your position. You're better off taking cover and waiting for him to leave.
And as far as I'm concerned, killing = murder. If it makes you feel better that it's legal to take a human's life away from them, that's fine. But it makes no difference to me, if you intentionally kill somebody, you've murdered them.