Well spotted. Because they weren't really guilty - in their twisted world view it was society that was guilty for making them do those crimes, so they had to be released to take vengeance on the unjust society. There's some good stuff about this in Dostoevsky's Crime and Punishment; if a criminal blames their crime on society rather than their own choices, then you may as well execute them because they will take their vengeance at will when they're released.