The "Lindsay Clancy defense", AKA not guilty by reason of insanity, has roots going back thousands of years, and was used as early as 1844 in the US in the murder trial of Abner Rogers, Jr. In 1982, John Hinckley, Jr. used it after shooting at the president. Others have used it even after the post-Hinckley reforms. Yet society hasn't crumbled.
Saying "He had no free will" doesn't mean "The state has no right to confine him away from people he might hurt."
Saying "He had no free will" doesn't mean "The state has no right to confine him away from people he might hurt."