A good (not for-profit) criminal justice system is built upon the idea of reintegration. If you 100% cannot reintegrate a(n ex-)criminal, then sure, relieve society from that burden.

EXCEPT that you can never be 100% sure of either guilt nor lack of reintegration, so lifelong internment is the only solution.

To quote Gandalf: "Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgement. For even the very wise cannot see all ends. I have not much hope that Gollum can be cured before he dies, but there is a chance of it."