I study psychology as a hobby, and everything I've ever read says that these sorts of people have one thing in common.
They think nothing is EVER their own fault. If they lose their job, it's the bosses fault. If they spill their coffee, it's the coffee company's fault. If they can't get a date, it's everyone else but THEM.
So when some of these people get angry enough and decide that the whole WORLD is to blame for whatever stupid hole they dug themselves into.... tragedies like this can happen.
It's not that he was ill, and VERY rarely does it turn out that the person has a brain tumor that gave them no choice but to be violent. It's just losers who can't accept any responsibility for BEING a loser.
Don't let it scare you too much, or become afraid of going out. The sad truth is yes it happens, but more people are killed by bad weather every year and we still leave the house.
I got a thumbs down for that from someone? probably a spree killer around here not liking being called a loser.... LOL