I'm not so quick to believe in the stories of these shootings the past year...

But if they are being reported accurately...then I think these shooters are just assuming that's what you do. They see it on movies, in video games. When planning for it, they think of what gear they will need.

They are not thinking entirely logically, like "I will die anyway."

Primarily: it looks badass. So that's probably why they do it. If they really did it.