A jury sentenced Dzhokhar Tsarnaev, 21, to death by lethal injection for his role in the deadly 2013 Boston Marathon bombings. CBC's Paul Hunter delves into the past of Dzhokhar and his brother Tamerlan, who died during a gun fight with authorities, to glean what makes a killer.

The brothers grew up in Chechnya before unrest forced them to flee to Krgyzstan.

Dzhokhar Tsarnaev, 21, was sentenced to death by lethal injection by a jury last week. (FBI/Associated Press) In 2002, the family moved to Cambridge, Mass., and seemed to embrace the American dream. Their parents envisioned Tamerlan as a boxer for the American Olympic team and Dzhokhar studying at an esteemed institution like Harvard University.

But those dreams remained distant, and Tamerlan remained an outsider unable to box for America thanks to a rule change, unemployed and married with a child. A frustrated Tamerlan went back to visit Dagestan.

Meanwhile, Dzhokhar earned poor grades and massive debt in college.

When Tamerlan returned from Dagestan, he reconnected with his floundering younger brother, and the pair made plans to detonate bombs at the Boston Marathon finish line.

Watch Paul Hunter's documentary at the top of this story to learn more about the brothers behind the bombings.