We seem to be thinking in the same direction. The American press whines for more knee-jerk centralization of police and government. Meanwhile, we know that far-away, central authorities don't protect anything, except their own interests. Without real power and responsibility for it, local communities descend into chaos.
And while the idea of “vigilante justice” is extreme - there are other ways that the local community might have prevented this. Mainly by being aware of this “kid” (from years ago it sounds like they were), and then doing something on the local level. It is hard to fathom with almost 40 contacts with the police that something wasn’t done.
In my city our church (and now lots of other churches) are heavily involved in the public schools. We run their sports programs. Have mentors for the kids, etc. It is unbelievable what an impact that has made on the schools. Stuff like suspensions going from 100+ down to 12 in a year, grades going from 30% passing to 80%, etc. I’m guessing that a nut-case like this kid would get found out and either received the help they needed, or at least some steps taken to keep him from being a danger to others.