The rehabilitation thing looks like it would work IF there was a way to guarantee that the criminal has no desire left to commit crimes.

Forgive me for the obscure childish reference, but

When Man-Ray was released accidentally by SS and PS, he returned to his evil ways. However, using MM and BB’s Tickle-Belt, SS and PS were able to defeat Man-Ray. Even after Man-Ray faked goodness to escape the clutches of the Tickle-Belt, its effect lingered whenever he tried to be evil. Thus, Man-Ray’s desire to do evil was completely suppressed. If this was possible in reality, then it WOULD be more effective than simple death.