I still haven't had time to really look at this situation, but I have to address this nonsense about his psychiatric history: it's irrelevant.
He didn't do this because he was on Prozac, or he was Bipolar. Look at it the other way: are we going to say that people with bipolar are more likely to go homicidal? If so, should we do a Kansas v. Hendricks for bipolars? (in which the Supreme Court said it was ok to lock up pedophiles indefinitely, even in the absence of a crime, since "pedophilia" makes you a priori dangerous.)
If you want to really understand why he did it that way, you have to find out what article of media he was imitating. Take the photos, the manifesto, and google it until you find the movie the handguns came from; the book (or comic book) the manifesto came from. He didn't come up with this stuff on his own, he is imitating something. For Klebold and Harris it was the Matrix and Doom. What movie is he imitating? Find it.
Because it isn't about mental illness, or genetics. It's about identity, it's always about identity, and sometimes the identity you choose doesn't work out that well. So, emergently, you grab an identity which has appeared to work-- you imitate a movie, a game, a comic.
I'm not saying movies made him do it; I'm saying he was looking for an excuse to do it, and he went through the usual catalog: movies, comics, games. Come hell or high water, he was going to kill someone. But in terms of prediction, the operative question is, if this guy goes homicidal, how will he do it? He didn't strap explosives to his chest, not because it wasn't available, but because it didn't match the identity he wanted to have-- that he got from a TV show or movie.
Ismael Ax, handgun to the head, hammer cocked like a bat to the right, knife to the neck-- all those stills from his video clips you see on CNN aren't random, they're a specific imitation of something else. Find the thing he was imitating, and you have found him.
Because he didn't exist, that's the problem. He picked an identity, and no one liked it, it backfired- no chicks-- so he moved to plan B: pick an identity that absolves him of the guilt of shooting 30 people.
Stop looking in the DSM. Start looking in IMDb.