While I agree with the theological argument, I wonder if that holds much weight in a non-theocratic, pluralistic society. By which I mean, arguing boldly from one’s religion isn’t persuasive in the civic square.
If I may be secular for a moment, I think the strongest argument is simply that a murderer has surrendered any claim on societal inclusion or protection by breaking the axiomatic pact necessary for all society. We could either thus kill him, or drop him on an island of feral convicts a la the Ray Liotta flick No Escape. Except society’s under no obligation to pay his air fare either.