You know, honestly what can they do with him.

If they executed him they might martyr him.
If they lawfully sentence him, he might get parole in 20 years.
If they found him insane it diminishes what he did as the actions of some nut, and it means 70+ people died for no reason.

Not convinced his victims families get any real redemption from all this. Maybe the best thing to do now is ignore him. He is in prison now and cant hurt anybody else.