Be specific: which disorders shoudl trigger involuntary committment? Keep in mind that there is zero causality between depression, anxiety disorders, bi-polar disorder, and even schizophrenia and homicidal behavior. Zero. And, statistically, schizophrenics are a greater danger to themselves than to anyone else. And good luck in this day and age and our discrimination laws and other protections for the mentally ill in successfully pushing through a rebirth of involuntary institutionalization.
Instead of going down that fruitless road, maybe you should do more advocating of segregating violent people, young and old, from society? I see this shooter was 18. Does anyone doubt that someone will dig up a long trail of violent and disturbing behavior that was met with indifference or cover-up? That's the safest bet, a long track record. The other thing with most all of these types of murderers is chronic abuse of mind-altering drugs.
Maybe get serious about removing violent kids from society earlier instead of waiting for them to kill or maim and then taking care of the situation?