It is very easy to describe the alleged Marathon bombers as monsters. But understanding their motivation requires us to examine human needs for belonging and respect.

By all accounts, Dzhokhar Tsarnaev, the younger of two brothers suspected of having perpetrated the Boston Marathon bombings, was a good kid, a bright young man and hardly the type of angry malcontent you'd expect of a terrorist. He had been an honors student at Cambridge Rindge and Latin High School where he starred on the varsity wrestling team, and was awarded a city-funded scholarship for college tuition. One neighbor even characterized the 19-year-old as guy who gave off "a good vibe."
Dzhokhar's 26-year-old brother, Tamerlan, was an engineering student at local community college according to his own account in a Boston University student publication from 2010 quoted by AP. Although Tamerlan, according to the same source, had felt friendless here in America, he had apparently given no one any indication of having the potential or the desire to commit an extreme act of mass violence. Tamerlan's boxing coach described him as a nice guy and a piano player who often brought his younger brother to boxing practice.
Given their unremarkable lifestyles and reputations, how and why could the Tsarnaev brothers have engaged is such diabolical crimes? How could these young men have ruthlessly murdered and maimed spectators at Monday's Boston Marathon and days later fatally shot an MIT police officer?
The key to motivation may lie not just in some anti-American sentiment but in the brotherly bond itself. Individually, neither may have been prone to murder, but together they brought out the very worst in each other, both in ideology and behavior.
Time and time again, we have seen horrible crime sprees perpetrated by partnerships, including brothers, cousins, spouses, and even parents and their children. The leader, which in this instance would appear to be the older brother, is driven and emboldened by the admiration of his protégé and "foot soldier." The follower tends to be inspired by gaining the approval of his mentor.
Examples of such partnerships abound: Brothers Gary and Thaddeus Lewingdon killed 10 in Ohio. Gerald and Charlene Gallego kept a series of sex slaves in California, eventually killing 10 women. Bruce, Norman and David Johnston operated a crime ring in Pennsylvania, killing those they suspected might rat to the FBI. D.C. snipers John Muhammad and Lee Malvo had a pseudo father/son relationship. Cousins Angelo Buono and Kenneth Bianchi killed 10 in the Hillside Strangler crime spree.
Why did the Tsarnaev brothers, despite having allegedly committed Monday's horrible bombing, leave unharmed the man whom they reportedly had carjacked and robbed of his ATM card? Fortunately for this survivor, a personal connection had been formed with the two suspects, however brief. He would have been viewed by the brothers as a human being, whereas all those killed or injured at the marathon would have been seen as anonymous faceless targets. Criminals often find it easier to kill those with whom they have no personal contact.
It is very easy to describe the marathon bombers as monsters, and unquestionably their deadly deeds were monstrous. Although many with good reason have referred to the suspects as "animals," understanding their motivation requires us to examine human needs for belonging and respect that are fed in criminal partnerships.
James Alan Fox is the Lipman Professor of Criminology, Law and Public Policy at Northeastern University in Boston and author of The Will to Kill: Making Sense of Senseless Murder. http://www.amazon.com/Will-Kill-Making-Senseless-Murder/dp/0131375679