I have the feeling that this guy is just outright trolling. He obviously doesn't see the Gritty Grapnel tragedy the same way the most of us do and just wanted a reaction, unless, of course, he didn't expect this kind of reaction and posted it merely as a joke.
In other words, this guy isn't planning on shooting up a school at all, although he took it too far with the joke. It's like saying that Electrk is going to become a serial rapist because he released his Rape mod.