Fuselier and Ochberg say that if you want to understand "the killers," quit asking what drove them. Eric Harris and Dylan Klebold were radically different individuals, with vastly different motives and opposite mental conditions. Klebold is easier to comprehend, a more familiar type. He was hotheaded, but depressive and suicidal. He blamed himself for his problems.

Harris is the challenge. He was sweet-faced and well-spoken. Adults, and even some other kids, described him as "nice." But Harris was cold, calculating, and homicidal. "Klebold was hurting inside while Harris wanted to hurt people," Fuselier says. Harris was not merely a troubled kid, the psychiatrists say, he was a psychopath.

In popular usage, almost any crazy killer is a "psychopath." But in psychiatry, it's a very specific mental condition that rarely involves killing, or even psychosis. "Psychopaths are not disoriented or out of touch with reality, nor do they experience the delusions, hallucinations, or intense subjective distress that characterize most other mental disorders," writes Dr. Robert Hare, in Without Conscience, the seminal book on the condition. (Hare is also one of the psychologists consulted by the FBI about Columbine and by Slate for this story*.) "Unlike psychotic individuals, psychopaths are rational and aware of what they are doing and why. Their behavior is the result of choice, freely exercised." Diagnosing Harris as a psychopath represents neither a legal defense, nor a moral excuse. But it illuminates a great deal about the thought process that drove him to mass murder.

It begins to explain Harris' unbelievably callous behavior: his ability to shoot his classmates, then stop to taunt them while they writhed in pain, then finish them off. Because psychopaths are guided by such a different thought process than non-psychopathic humans, we tend to find their behavior inexplicable. But they're actually much easier to predict than the rest of us once you understand them. Psychopaths follow much stricter behavior patterns than the rest of us because they are unfettered by conscience, living solely for their own aggrandizement. (The difference is so striking that Fuselier trains hostage negotiators to identify psychopaths during a standoff, and immediately reverse tactics if they think they're facing one. It's like flipping a switch between two alternate brain-mechanisms.)

None of his victims means anything to the psychopath. He recognizes other people only as means to obtain what he desires. Not only does he feel no guilt for destroying their lives, he doesn't grasp what they feel. The truly hard-core psychopath doesn't quite comprehend emotions like love or hate or fear, because he has never experienced them directly.

"Because of their inability to appreciate the feelings of others, some psychopaths are capable of behavior that normal people find not only horrific but baffling," Hare writes. "For example, they can torture and mutilate their victims with about the same sense of concern that we feel when we carve a turkey for Thanksgiving dinner."