Here are three clues to what contributes to the attitude of a killer--chillingly, illustrated in the aftermath of the disaster.

The killer's roommate: "If I was told before he was depressed or suicidal, I definitely would have kept an eye open ... I definitely would have tried harder to be his friend or know a little bit better."

Dr. Hurd: You can't be friends with a nihilist hell-bent on destruction. Evil is not the same as emotional conflict. If you still don't understand this in the aftermath of the tragedy, then you're never going to understand it; and the way is paved for another one, and another one after that. Killers flourish in a psychological atmosphere where their potential victims think like this. This man didn't need counseling, and never would have benefited from it. He needed to be stopped, back when he was stalking women and making threats, and otherwise violating the individual rights of those on a campus.

The killer's creative writing teacher: "He was so distant and so lonely," she told ABC's "Good Morning America" Wednesday. "It was almost like talking to a hole, as though he wasn't there most of the time. He wore sunglasses and his hat very low so it was hard to see his face."

Dr. Hurd: Many people are lonely. They can't find people with whom to connect; they can't find people on their "wave length," if you will--that is, people who share their philosophy or sense-of-life. Yet they want this connection, and they generally seek it out. Cho didn't want it or need it. He only wanted and needed to destroy. Don't try to understand it; it's too irrational and sick to contemplate. But, at the same time, don't try to relate it to the realm of the reasonable, either.

The killer's poetry teacher: "I know we're talking about a youngster, but troubled youngsters get drunk and jump off buildings," she said. "There was something mean about this boy. It was the meanness — I've taught troubled youngsters and crazy people — it was the meanness that bothered me. It was a really mean streak."

Dr. Hurd: Come on, professor. You can say it. Go ahead, I dare you. Say it. He was EVIL. He was BAD. He was not quantitatively different from your average, stressed out college student...he was qualitatively different. He acted with choice, no less so than the 9/11 killers, the Columbine killers, or the Oklahoma City killers. It's not mental pain or anguish. It's hatred and evil.