I'm not sure this is mental illness but a creature whose bad behavior has been indulged. It does not matter though since peaceful society can't be maintained with him in it. If he "triggers" in the slammer he will get a taste of his own medicine.
Liberals don't understand boundaries and the controls necessary to have them and that rights don't mean anything you want to do is OK no matter who they hurt. I don't think it even enters their heads. How would they respond if it was they who got beat to a pulp by this animal without self-control or boundaries?
You might want to read his story at the link I posted.
He was never "indulged", he suffers from Level 3 Autism that the parents have had to deal with his entire life.
He is not an evil person and prison will do nothing for him. He needs skilled, medical supervision, not prison guards.