When I was in high school, I once had to debate in favor of capital punishment. There is no argument to be made that it is a deterrant -- it is not. The stats aren't there to show that.
The best argument is that it permanently removes from society someone who has proven themself unfit to be here.
I'd kill them all. Every one.
Coming from someone who is flirting with becoming a pacifist (Mennonite), that is somewhat rude. But I'd still do it.
Life will be better without them.