The bottom line is that the world is a better place without him. If he were left alive someone would have to waste their lives guarding him. Who wants to pay for that?
While the idea of keeping him alive to be tortured sounds like a "fun" idea, in the end it would only have a negative effect on the people that acted on their urge to abuse another human being.
Personally, I believe that we all will stand before God and have to answer for our actions so I don't need to worry about the amount of punishment he receives.
