Here's a hypothetical. Say you're on a desert island with 100 people. One guy goes nuts and decides he wants to kill everyone else. He manages to kill a few, but some men manage to restrain him. Now, what to do with him? The rest of the people could kill him, or they could imprison him. The problem is that would require one guard constantly, which would mean enough less labor that one of the islanders would starve to death each year. What's the moral choice?