It reminds me of a Manchurian Candidate scenario. It doesn't help that the number of deaths was 33 (32 + the shooter). I am sure some of those who are here realize the significance of the number 33. I am sorry to say I don't buy this story one bit.
You don't think one guy could kill more than 30 people, is that what you're saying?
If you do so, think that way:
You don't have anything to lose, anyway. Most people annoyed you at school because they mobbed you because you are an emo, or whatever. Just someone "who doesn't fit in". It doesn't matter why, at all. The point is, that it seems like he wanted to have revenge.
I am no educationalist and I certainly don't want to play like some skilled psychologist-games, but I think he was just pissed off and shot bullets into the classrooms, randomly.
He didn't give a shit about what would happen, he would be dead in "a few minutes" anyway.
I don't know how many pupils visit your school, but we've got around 2000 here, that's a bunch.
Now imagine one floor in a building. When I go through the first level, I have like 10 rooms on my way until I reach the staircase.
If I'd open up every door I cross and shoot a pistol or an automatic gun for like 10 seconds, each door, I certainly would have about 30 people killed or at least hurt deadly.
It's not that unrealistic, you know. And I
do believe the guy could have done it without regrets.
Mop

/€dit: I sound like a psycho, sorry. I was just trying to make clear that it's not impossible to do "that kind of thing".