What Motivated the Elder Chechen Brother to Turn to a Life of Jihad? I Say We Blame His Mom

She wanted to keep him away from bad influences and so she turned him toward Islam:

Once known as a quiet teenager who aspired to be a boxer, Tamerlan Tsarnaev delved deeply into religion in recent years at the urging of his mother, who feared he was slipping into a life of marijuana, girls and alcohol. Tamerlan quit drinking and smoking, gave up boxing because he thought it was in opposition to his religion, and began pushing the rest of his family to pursue stricter ways, his mother recalled.

"You know how Islam has changed me," his mother, in an interview with The Wall Street Journal in Makhachkala, Dagestan, says he told her. 

If he'd balked at mom and stuck with his life of booze, broads and boxing he'd probably be alive today.