To the point, mental illness is useless as a prospective indicator of potential mass shooters, since the vast majority of people with mental problems do not become one.

That is the issue, it's not about blaming anyone or anything, it's about getting people help before they snap. It's about having that option to get the help they need, because right now, for us in the USA, it's really not an option at all. It can take months for people who need help to find it, and even then, if they're insurance won't cover it and they can't afford it, their fucked.