We need to be prepared to kill them in self-defense. I don’t believe in outright killing someone just for their beliefs, no matter how vile. Certainly keep a close eye on them, exact the full penalties of law on them, but killing is open warfare, and that’s not a bell that can be un-rung. You have to be damn certain that’s the path you wanna walk, and it’s rarely a good choice, even when it’s the best choice available.