To my mind, the strongest argument against capital punishment has always been the possibility that an innocent person would be put to death. The principle propounded by the jurist William Blackstone in the 18th century, that it is better 10 guilty men walk free than one innocent man be convicted, is a powerful one with its roots in the Old Testament. But there is another way of looking at this that has equal force: capital punishment ensures that a murderer cannot kill again.