Neither the Chos, nor the Columbine families ever talked freely with the press about their sons' actions. But in 2004, on the fifth anniversary of the tragedy, Tom and Sue Klebold, who still live in Littleton, Colo., responded to an article in the New York Times.
The Klebolds told reporter David Brooks that they objected to the way their son had been described as "depressive" and blamed the toxic atmosphere of teasing at the high school.
But Cullen said that unlike Cho, who was not well-liked and kept to himself, Harris and Klebold had an active social life and were bullies, rather than bullied.
"We always get the wrong answer because we phrase the question wrong," Cullen said.
"Everyone says, 'Why did they do it?' That gets you in trouble. There isn't one thing to explain Columbine," he said. "Why Eric did it and why Dylan did it -- they are polar opposites. You can't fuse it into one.
"It's the same thing with school shooters," he said. "We still go the same route and look for a profile and think we've got one -- outcast, loners and bullies. In two-thirds of cases, they don't apply. There are three or four or five profiles."
According to former FBI psychiatrist Frank Ochberg, who worked in hostage negotiations in the 1970s, Cullen's book "hit the nail on the head."
"The general public has its own idea about evil and how it gets created, distilled and powered," Ochberg told ABCNews.com. "We have so many archtypes."
Harris was a "budding psychopath, a person without a conscience," he said. "He got his satisfaction by dominating."
"Psychopaths don't feel guilty because they are blind to guilt," Ochberg said. Harris also had sadistic tendencies, which propelled him to "seek vengeance."
Klebold, on the other hand, was depressed, with pent up anger and "mood regulation problems," but together, they had "violent creativity," Ochberg said.
"Eric needed Dylan's emotionality and impulsiveness, and Dylan needed Eric's cold psychopathy," according to Ochberg.
While Klebold longed to end his life, as seen in his journals, for Harris, suicide was not a concern, according to Ochberg.
"His life wasn't as important as his appetite," he said. "He turned a comic book fantasy into reality. The purpose was not to kill himself, but it was an option, He needed power."
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