didn't see this thread before...
but Winterreverie has a point. People get hungry for blame. They were hungry for blame from the 1st hour after this horrible tragedy. The way the media went after the cops was just tasteless and... vulture-like. Eventually they went after the essays.
Of course you can't take someone's essays as a sign of mental disturbance. I thought his were badly written, badly developed. And the anger and twisted logic inherent in his characters could be deliberate. But if they're not deliberate, would that, then, be a sign of a mentally disturbed and potentially dangerous person? I don't know.
From day one I tried to understand how a person can do such horrible acts. I mean, our human race has it's dark side, where people hurt others for personal gain. But here, there was no gain. This goes way beyond our 'dark side'.
I wrote an article on my blog where I theorized on what makes such a mass murderer... but I won't ask you to go and read it (You're welcome to, but I'm not here to plug my blog

)
So the long and the short of it is that imho, it is not just anger and a penchant for solitude that makes a mass murderer.
Some people need support, and find it in life. They find it in religion, in fantasy, or in any other form of support-system available in our culture. Some people don't need it at all...
And maybe there are those who do need it but can't find it. They end up feeling cut-off from the society around them. Resentfull of it, cause they don't understand it. You add to that anger and solitude, you got trouble.