Mental Health Issues, that is all.
Nice words bro.
Now how do you want to treat them, and how do you want to prevent them acting out in that way?
You have two options. Like the greek, that included them in their daily lives gave them respect and honored them, or like the english, who locked them away, never to see the light of day again.
(read Michel Foucault)
edit: I have to state, that we are talking -before- they do something like going on a shooting spree or rampage.
This is the other part we havent looked at. Whenever we say "how can this be prevented", we (left leaning folks) talk about socialization efforts essentially.
All that your thought process is circling around is family upbringing, and then prouding yourselves over not offering your children a broken home experience. Again, thats pretty low bar. I dont know, why you folks even like to talk about is so much. Again - its probably a "4/5 stars, I bought it, so mine is better" thing - that has really not that much to do with the issue at hand.
(Whose is the better parenting method - usually doesnt play that much of a role in who becomes the next mass shooter.)
Again, look at other countries, do you think, that your parenting methods on average are 300x worse?
In any case - that behavior triggering, is always a combination of nature, and nurture (upbringing), which I tried to offer you a bit of a deeper look into (what are the motivators, what are the logic jumps, how does the internal logic look like, how does it develop..).
So just giving them a name (clinically insane), which we do, btw - solves nothing.
Next thing you come up with is "giving them proper attention", which means calling up some institution. Guess what, thats failing as well.
So what do those words mean in that context? Look away and do nothing. Single out some individual, just because we dont like them. Then call a phone number, if they overreact. Those things develop over time. You maybe could have had 20 opportunities, to do something, to integrate such a person somehow. But instead you uttered that sentence.
I wont even call you dumb, because you really are the majority position on that - but you arent adding to any solution. You know that, right?
In the end you are just namecalling. But thank you for bringing it up in this thread, so I could address it.
edit: To condense it down even further, before they do something we call them silent, but very friendly people - and afterwards we call them clinically insane. What a help indeed.