A man sentenced to life in prison with no chance of parole, is a free man, in that there is nothing he can do in his life to make it worse. He could kill inmates, prison guards, etc., and what extra punishment could you give him? None. A man like that is a danger to the prison guards, who guard him, because he has nothing to lose. That, and the interesting angle brought up Mark Klass and Debra Saunders are why the death penalty works, even when it doesn't execute people.