"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."
According to FBI trainer Lanning, psychopathy and psychosis can overlap, but the public wrongly uses the terms interchangeably.
Psychotics are mentally ill, delusional and out of touch with reality; psychopaths can be "wheeler-dealers and manipulators," he said.
Most psychotics are not violent, but their nature is unpredictable, he said.
"Neither is necessarily a killer," said Lanning. "But society tends to focus on those common violent crimes."
Whether psychopaths -- sometimes called sociopaths -- lack a moral compass is up for debate, according to Lanning.
"They have a conscience," he said. "It's just that it's their own, not society's.
"A sex offender may kidnap and rape and mutilate women, but if you put him in prison next to the guy who fondles children, he thinks he's a sick pervert," he said.
Some sub-cultures admire the character of psychopaths.
"If you're a con artist and cheating people out of their savings, the best character to be is a psychopath," Lanning said.
When raised in a nurturing family, they tend to be thrill-seekers -- race car drivers and mountain climbers, "which is more acceptable," he said.
In fiction, they are self-focused characters like J.R. Ewing from television's "Dallas" and Scarlett O'Hara from "Gone With the Wind."
Still, one of the lessons of Columbine and Virginia Tech is understanding the complexity of the human psyche and the difficulty of identifying which teens will cross the line and become a killers.
"Remember Charles Atlas?" asked Lanning, who cites a comic book ad that featured a 90-pound weakling who gets sand kicked in his face, builds his body up and seeks revenge.
"The idea of avenging through physical force for slights against is the age-old dream of adolescent boys," he said. "You are an outcast, you get picked on and you want to get even."
ABC's information specialist Nicholas Tucker contributed to this report.