About a decade ago, while visiting the farm....my dad made an observation that concerned the late 1930s in Alabama.
In those days....a relative could write up a letter detailing how so-and-so relative was crazy....then two local folks would 'swear' (full oath in front of judge), and then some judge would order the county sheriff to bring in so-and-so for a face-to-face meeting. The judge would assess the threat-level, and then make the decision to confirm that guy or gal to a real mental facility.
As my dad discussed the matter....he noted with a grin that most young folks took this as a humorous moment and joked about having such-and-such relative sent off. For folks over forty, it wasn't joked about much.
I look at the Florida shooting episode, and this punk kid, with some descriptions given of him and his behavior....and in the 1930s era, he would have been packed up at age 13/14....and likely sent off to be evaluated for a couple of months. Maybe he would have learned to grasp reality and earn his way out.....or maybe he would have stayed permanently in such a place.
In the 1950s and 1960s....everyone decided that the mental institutions were not good places, and they more or less went away. If places exist today....it's more or less a place where you go for a couple of months until they figure out the right drugs to give you.
The plain truth is....there's probably over 3,000 folks just in the state of Alabama today....who ought to be in a mental institution. If you count up folks who have made violent threats to relatives....maybe another 20,000.
Across the entire US? You could be talking about locking up 300,000 people very easily. I just don't see the public accepting that.