Mental health issues don't usually appear suddenly overnight. There are usually warning signs. Sometimes it take a conscious effort of will to ignore them. Somebody could have spotted this about to happen before it happened, given the inclination; and somebody could have prevented it from happening, given the resources.
There's no reason why he couldn't have been found gainful employment in a field that would not have exposed him to dangerous situations. There are plenty of non-public facing rôles in laboratories and warehouses, that don't really require any empathy. Someone who genuinely lacked the capacity to understand what other people might be feeling, would never need to be put in a situation where they might cause harm to someone vulnerable; and someone who had just been so badly treated that they never learned what not suffering feels like would have the chance to learn.
But if we began treating our most vulnerable like human beings, then the not-quite-so-vulnerable might start getting jealous, wanting their potential recognised too. Finding jobs for disabled workers that suit the things they can do (which usually happens to be most of what everybody else can do) requires an investment. To the middle classes, it looks patronising; and to other workers, it looks like preferential treatment. It's much easier to ignore the problem and pretend it only affects other people. Paying out benefits seems to work out cheaper than investing in creating jobs. And a large pool of unemployed workers also tends to keep wages down. The ruling classes never factor in the social cost of people not having regular paid work. (I can't speak for anyone else, but my day job is literally the thing that keeps me sane. I have to get out of the house five days a week, and I have to earn the money I get as opposed to being given it. It also helps enormously that I genuinely enjoy the fuck out of what I do for a living. Otherwise, I could be lying in bed all day, smoking myself into oblivion; only ever leaving the house to sign on or score more smack. Well, why not? It's not as though I have to pay rent or a mortgage, or look after any dependents. And it would be very easy to convince myself that the world beyond the front door was a very scary place indeed. I just got very lucky, where thousands of others didn't, and I'm grateful for that every day.)
For the rich, none of this is a problem. They have the resources they need, and the means to pay for them. They can afford private medicine. They can lock themselves up in gated communities, where they needn't be bothered by poor people. And when something happens that creates a scapegoat, that is a positive benefit; as it basically means the working classes will turn on their own kind instead of them.
Without for a moment excusing what he did, I still think this guy was not just let down by the system, but let down on purpose.