Like a stereotype?
No, it fits the profile of mythology. It explains the reason for things happening that are completely unreasonable. Horrors that we can't wrap our minds around. A stereotype is the product of a lazy or shallow mind. A mythology is something you strive to understand but come up empty-handed over and over again something that defies reason but exists anyway.
But mythologies are untrue explanations.
Yeah, but they remain until the real explanations come. And these lunatics who blow their tops unexpectedly one day I don't think that's so easily explained. It could happen at any time, and to any of us. Really, think about it: Those people who get killed, they haven't done anything wrong, and they're your mom or your daughter you know, the people you love the most, or somebody does and they're meaninglessly dead. It's beyond tragic. There's no pacifying that kind of horror, or explaining it, except that there's a cruelty in the world that can't be negotiated with.