I'd take the gun, kick out the back of his legs, hold the gun to his head, act like I'm going to kill him, wait untill he brakes down...
Then I'd call the cops and keep him there, crying for his life. Let them take him away. Maybe vist him a few times afterwards, see how he's getting on. He might change, might show remorse, he might not. If the former, good. If the latter, he'll learn, next time he goes back to kill again.
... Sure, it isn't particularly bad-ass, but it's probably what I would do. Maybe you don't belive me when I say that, given that the dude just killed my parents - but other people have gotten over their family being murdered, and eventualy made friends with the killer. No, really. If they can do it, I can't see why I shouldn't.
Then I'd call the cops and keep him there, crying for his life. Let them take him away. Maybe vist him a few times afterwards, see how he's getting on. He might change, might show remorse, he might not. If the former, good. If the latter, he'll learn, next time he goes back to kill again.
... Sure, it isn't particularly bad-ass, but it's probably what I would do. Maybe you don't belive me when I say that, given that the dude just killed my parents - but other people have gotten over their family being murdered, and eventualy made friends with the killer. No, really. If they can do it, I can't see why I shouldn't.