My favourite American politics for Australians TV show made this point too in fact, and very well. At the time I couldn't find the clip or I would have posted it in here.
As to the "opportunities to intervene" - obviously it's easy to analyse one of these events after the fact and see all the warning signs, but this guy was known to the FBI, right? I wonder how many such people they have on their books (so to speak), what powers they have to monitor them and whether they could even effectively intervene. Maybe they can't, or maybe they already do and it happens all the time? Who is ultimately responsible for saying "hey, this guy looks like he needs a hug"?