In finishing Dave Cullen's very well written, "Columbine," I am reminded of something that seems to me true: there are no complete explanations for actions which are evil. One chooses an evil action if and only if one chooses a lesser good over a greater and there can be no complete explanation for why one would do such a thing. In the end, the will wills because it wills.
From the "basement tapes":
Dylan: "Hey mom. Gotta go. It's about a half an hour before our little judgment day. I just wanted to apologize to you guys for any crap this might instigate as far as (inaudible) or something. Just know I'm going to a better place. I didn't like life too much and I know I'll be happy wherever the fuck I go. So I'm gone. Good-bye. Reb..."
Eric: "Yea... Everyone I love, I'm really sorry about all this. I know my mom and dad will be just like.. just fucking shocked beyond belief. I'm sorry, all right. I can't help it."
Dylan: (interrupts) "We did what we had to do."
Eric: "Morris, Nate, if you guys live, I want you guys to have whatever you want from my room and the computer room."