Edgelord: If you let him live, you're as good as killing everyone he later kills!
Me: Am I then responsible for all the lives he saves in the process?
Edgelord: What? What the hell are you talking --
Me: If he kills someone who would then kill five people, those five people are going to die if he doesn't kill the first person. If I kill him, I've killed those five people. And what if one of those five people will save countless lives in the future? By killing him and thus them, I've killed countless lives.
Edgelord: You can't know --
Me: Oh, so this morality play you've set up only works the way you want it to? How convenient. Go play with someone else. I have control only over my own choices and do not seek anything else. Anyone who does seek that is a tyrant in the making, and I would inevitably be one of the bodies in the foundation of any utopia they seek to create.