Dylan Klebold, the depressed senior at Columbine High School, had Eric Harris. Washington, D.C., Beltway sniper Lee Boyd Malvo had John Allen Muhammad. And just last week at the Boston Marathon bombings, suspect Dzhokhar Tsarnaev had his 26-year-old brother, Tamerlan Tsarnaev.
Klebold, Malvo and Dzhokhar Tsarnaev all might be seen as more passive teenagers who were attached to more assertive mentors whose actions had deadly consequences, according to mental health experts.
MORE COVERAGE: Boston Marathon Bombings
On April 20, 1999, Columbine High School 18-year-olds Eric Harris and Dylan Klebold slaughtered 12 students and one teacher in a 45-minute rampage before killing themselves. Post-mortem, experts diagnosed Harris as a psychopath.
During three weeks in October 2002, military veteran Muhammad and 17-year-old Malvo killed 10 people on a rampage throughout Washington, D.C., Maryland and Virginia. Muhammad, at 41 and a father figure to the teen, had planned to train youths to carry out mass shootings. He was executed by lethal injection in 2003. Malvo is serving six life sentences in prison.
"Very often, these are pairs of perpetrators, one dominant figure who has psychopathic tendencies and the other a dependent figure," said Dr. Igor Galynker, director of the Family Center for Bipolar in the psychiatry department at Beth Israel Medical Center in New York City.
"It's not accidental," he said of the attraction to the stronger figure. "But it's almost opportunistic in the negative."
Attachment issues and the anatomy of the immature teenage brain can create a killer, according to Galynker.
"In terms of attachment, Harris was psychologically a classic psychopath," he said. "And Klebold was a depressed loner who needed a role model and an attachment figure and acceptance. … Harris happened to drag him into his orbit."
In the case of Dzhokhar Tsarnaev, it may be too early to know, but Galynker said someone with an insecure attachment would "likely become psychologically dependent on someone who accepted them and loved to get close."
TIMELINE: From Terror to Manhunt to Capture
Teenage behavior, as every parent knows, is marked by impulsiveness and the inability to see the consequences of actions.
Now, science has confirmed that the human brain is not fully developed until the age of 25. The brain develops from back to front – from the emotional limbic system first to the rational frontal precortex last.
As authorities try to find answers as to why the Tsarnaev brothers, both well-educated American citizens with talent and promise, turned to terrorism, online forums have debated young Dzhokhar Tsarnaev's culpability.
New Yorker editor David Remnick painted a sympathetic portrait of the brothers.
"I think this had to do with his older brother," he wrote. "Unless he was some sort of sleeper agent, I think his brother had a pretty strong influence. Tamerlan maybe felt like he didn't belong, and he might have brainwashed Dzhokhar into some radical view that twisted things in the Koran."
Columnist John Nolte was outraged in the conservative Breitbart report.
"It is one thing to never forget, no matter the evil with which we are faced, our own humanity," he wrote, "It is quite another, however, and quite dangerous, to forget that we are dealing with evil. Such a thing not only invites more evil; worse still, it is a cruel slap in the face to the victims of evil."
He called Remnick's essay "poorly timed and even shameful attempt to define evil by something other than evil." He argued the brothers "made a choice" that killed four people and maimed more than 100 more.
But psychiatrists like Galynker said that a 19-year-old like Dzhokhar Tsarnaev would have "difficulty with executive functioning, assessing risk and understanding the consequences of behavior." At 26, Tamerlan, despite his alleged terrorist motives, was "fully matured and [his brain] myelinated."
Myelination is the process by which a fatty layer, called myelin, accumulates around nerve cells. It is a key to healthy brain development.
Teenagers are particularly vulnerable to the influences of others because of the immaturity of the ventromedial prefrontal cortex. That is the part of the brain where social consequences of actions are weighed, where planning and forming of strategies, as well as inhibiting behaviors, are rooted.
The human brain takes 10 to 12 years before it is generally developed. Structural maturation of specific regions and pathways is necessary for properly functioning cognitive, motor and sensory functions. The speed of neural transmission depends not only on the nerve cells or synapses, but also the connection fibers or axons.
Those structural properties include the thickness of the special insulation or myelin around many fibers. These continue to develop throughout childhood and adolescence and post-mortem neurological studies have shown they may not even fully mature until well after adolescence and into adulthood.
While those with psychopathic personalities -- a lack of conscience or empathy -- are beyond help, those with mood and attachment disorders, though vulnerable to the sway of a violent mentor, can be treated, according to Galynker.
"If someone had antisocial behavior at 7, torturing small animals on the weekend on a regular basis, nothing would change," he said. "But someone who is lonely and depressed and wants acceptance from anyone, even a serial killer, that person could change and probably would respond to treatment to some degree."
For that reason, some argue that the death penalty, in states where it is legal, be raised beyond 18, the age of legal adulthood.
Jesse Payne, director of undergraduate education at Corban University in Oregon, has created a high school program to teach young people about the immaturity of their brains to help them with better decision-making.
"We need to teach kids about their brain so they understand themselves," he said.
He is writing a book, "Change Your Brain, Change Your Life Before 25," which will be marketed to 15 to 25 year-olds."
"Bonding, relationships, feelings -- all of those things matter in the teenage years," said Payne. "Not until about the age of 25, and even 28 for boys, do you see executive function, empathy, impulsivity judgment and learning from their mistakes."
"In general, the brain of a 19-year-old is much more susceptible to influence because the prefrontal cortex is not fully developed," he said. "They make stupid decisions and also lack the ability to recognize the feelings of others."
The car industry, which lowers its rates at age 25, understands the risks, he said.
"They figured out long ago that people make smarter decisions late in life," he added.