He spent the next ten years researching the rampage for his book, “Columbine.” In painstaking detail, Cullen chronicled Eric Harris’s profile as a classic psychopath intent on killing as many people as possible. Cullen’s portrait of Dylan Klebold was more empathetic. His research showed Dylan as a depressed, suicidal follower — a sidekick who reminded Cullen of himself when he was a teen.