This just happened today. It's bad enough when taken on its own, but read the letter that this guy left. This is a coherent letter, in which his frustrations were comprehensive and specifically targeted. This letter shares a lot of similarities with stuff that I read on doomer sites every day. What the letter comes down to is, that the guy isn't crazy. His manifesto actually makes sense, and I think that's what makes it so dangerous.
Sorry to be alarmist, although this looks like it had a very low body count, this could have a significant impact considering that this guy came off as an average Joe who was pissed off at his country in the manifesto.