While I certainly disagree with Rat's theological conclusions...
I certainly agree that evil breeds more of itself. Abused children are statistically more likely to be abusers.
Previous school shootings have typically involved a group of societal outcasts who had easy access to firearms. I'm sure all of us have, at one time or another, felt like the "loser," or a "nerd," or some other such outsider. It hurts, it's depressing, and the treatment received can be infuriating. Nobody is ever justified walking into a school with a gun for revenge, and I'll never be able to understand the psychological disconnect that allows an angsty, pissed-off geek to override his conscience and rationalize mass murder as acceptible, but I can understand some of the motivations behind it. I think that;s the closest I'll get to really understanding these things.
If we were all a little nicer to each other, kids and adults, I think less of this horror would happen. But I won't hold my breath.