Hate them, as in Otaku? I thought maybe that was it, but it wasn't only otaku that he attacked, it seems like it was anyone who was around. And he said he's been there a few times so I wouldn't be surprised if he is one himself.
Well it seems he did intend to take his life, but backed out when he saw the gun?
Looks to me this guy knew very well what he was doing and wasn't mentally incapacitated, and just went on a murderous rampage.