Part of the problem - a big part - is how to reliably determine that any given individual is mentally ill to the point where they should be preventively locked up to protect society from them. How does one reliably separate the wheat from the chaff, and should we err to the side of being too inclusive - locking up some who, in actuality, are not dangerous - or of being not inclusive enough - failing to lock up some who, in actuality, are dangerous (and who, to boot, subsequently demonstrate that fact in fatal fashion)?
Red Flag laws, anyone?