> But there is something absurd about the possibility that a person with a demonstrated propensity for mass violence might be returned to society at large without any legal restrictions, psychological treatment, or period of supervision.

There is nothing illegal in wanting to kill people. There is nothing illegal in having radical religious (or other) beliefs. Free societies don't punish thought crime.

The only thing the law enforcement/legal system can legally and morally do, is to mark him as a potential threat (and increase surveillance) like they would with any other suspected terrorist.