It has been just over 10 years since Eric Harris and Dylan Klebold, two seniors at Columbine High School outside Denver, staged one of the bloodiest school massacres in history.

What were we to make of this? Armed robbers, terrorists, sex fiends, jilted boyfriends, and betrayed husbands all have a rationale, no matter how twisted or reprehensible. We are horrified, but we get it. Columbine seemed utterly senseless, and because it involved children, what parent didn't feel an urgent need to understand?

Could it have been prevented? Shouldn't the killers' parents have known something? The boys' friends? The school? The local police? Surely there was some colossal social failure here. Assigning blame distances us from the craziness. It allows us to believe it could not happen to a good family, or a good school, or a good community. At the very least it teaches us how to prevent its ever happening again.

With four boys of my own, I felt all these things. When a book tour took me to Denver in the week after the shootings, I spent some time with my uncle helping to erect platforms outside Columbine to display the floral arrangements piled outside. I was struck by how much the school and the community reminded me of my own outside Philadelphia.

In time, the killers' parents, their friends, the school, and the police would all catch portions of blame. There were theories about bullying, about high school cliques and fads, about violent movies and video games, about godlessness and the general decline of Western civilization.

But all this appears to have been misdirected. There is a new book about the episode, a remarkably detailed work of reporting by Dave Cullen, titled, simply, Columbine. It feels like the definitive story, and it disproves most of the prevailing wisdom. The killers were not loners. They were not crazy. They were not unpopular or bullied (both were actually bullies themselves). They had good parents. They were good students. They were reasonably popular.

They seemed dismayingly normal. The bad news is that there is no real remedy for what they did. The good news is that the monster who dreamed this up, Eric Harris, is an extraordinarily rare type. Cullen's research makes it clear that Klebold was the follower. He was a severely depressed, suicidal young man who got sucked into his buddy's vortex. One suspects that Klebold would have profited from professional help.

Not Harris. Here was a determined killer. He called his secret diary "The Book of God," which says a lot. Many teenage boys pass through a tiresomely narcissistic phase, and exhibit scorn for authority, parents, classmates, and society at large. Harris took these traits to exponential levels. He was God. Everyone, everyone was his inferior. His homicidal impulse was not rooted in rage, as the cliche would have it, but in contempt. His fondest ambition, which he outlined explicitly in his private rants, was to destroy humankind. Columbine wasn't personal, it was just the biggest thing he thought he could pull off.

Harris appears to have been a true psychopath, that exceedingly rare person who delights in killing and causing pain. He did not just lack empathy, he was eager to do harm. If his plans for Columbine had fallen through, who knows where his ambitions might have led? Cullen finds a clue in one diary entry, where Harris noted how fun it would be to lure younger girls to his room, then rape, kill, and dismember them: ". . . strangle them, squish their head, rip off their jaw, break their arms in half, show them who is god."

He hid these desires carefully. When he got in trouble, as he often did, he became a model penitent, reveling in private at how easily he fooled everyone, particularly his parents. He did not seem to appreciate that their love for him made them the easiest people to deceive. He worked out his plan with dedication, ingenuity, and enormous industry, procuring weapons, ammunition, learning to make bombs, testing them and training with the guns, drawing up detailed plans.

The plan was not about getting back at anyone - jocks, blacks, girls, certain cliques, Christians, bullies, etc. It was Eric's final act, a demonstration of his power and wickedness that would shock the world. He would blow up the school, then he and his buddy would pick off those who fled the inferno. He even had big bombs set to explode long after the initial assault, designed to kill emergency responders who raced to the scene. He imagined killing hundreds, virtually the entire school.

He failed. His homemade bombs fizzled, every one of them. He and Klebold then raided the school, trying to salvage their botched performance. They shot randomly until they seem to have wearied of it. Their climactic scene wasn't at all like they'd imagined, not like the big blowout scene on a Hollywood blockbuster with great explosions and surging music. It was just ugly and sad. There was blood everywhere, water from the school's emergency sprinkler system spraying down, the shrill scream of the school's alarms, and the cries and moans of their victims. In their final minutes, Harris and Klebold stopped shooting altogether, even though there were plenty of targets and they had more ammunition. They seem to have grown bored.

The massacre was not the fault of family, community, culture, or state, although none of these are ever perfect. It was the fault of Eric Harris, and his pathetic accomplice. Parents might feel more inclined today to snoop in their children's closets and hard drives, and police will no longer ignore teenage Web sites that threaten murder, but no amount of care will stop someone as determined and deceitful as Harris.

Much as we would like to, we cannot sustain a constant state of alert looking out for the next homicidal freak. It would be like checking over your shoulder every few seconds to make sure you aren't hit by a falling piece of space debris. Many of those around him saw warning signs, but who could have imagined the scale of his arrogance and ambition?

Just as this world produces an occasional saint, it coughs up an occasional psychopath. They stand at opposite poles of human behavior, and just as they taught me in my Catholic school days, we choose each day which way to lean. I have made it a point to thank my boys, all of whom are now grown, for never trying to blow up their high school. They laugh and no doubt regard it as another example of their father's peculiarity, but I could not be more sincere.

Mark Bowden is a journalist and author, most recently of "The Best Game Ever." E-mail him at mbowden@phillynews.com.