"As awful as his crime was, life in prison faced with what he has done is a better choice for everyone," said lawyer David Bruck in his opening statement at the penalty phase of Tsarnaev's trial yesterday.
The 21-year-old former student was found guilty this month of carrying out the 2013 bombings that killed three people and wounded 264 in one of the deadliest attacks in the US since 9/11.
The immigrant of Chechen descent was convicted on all 30 counts related to the bombings, the murder of a police officer, a carjacking and a shootout while on the run in April 2013.
"You will punish him and protect society at the same time," Bruck said.
More From This Section
There would be "no martyrdom" for the killer who claimed the attacks were to avenge US wars in Iraq and Afghanistan.
But the mastermind was older brother Tamerlan, 26, who was shot dead by police four days after the attacks, the defence argued.
Tamerlan was "consumed by jihad," going to Russia when Tsarnaev was a 17-year-old at high school and returning to the United States six months later, Bruck said.
A string of failures fuelled his self-radicalisation and he became an avid consumer of online extremist videos.
"If Tamerlan hadn't been in the picture, would Jahar have done this on his own?" asked Bruck, using the Americanised name favoured by Tsarnaev the younger's friends.
Speaking softly, Bruck portrayed an itinerant family history and culture where authority stemmed from the father and older brother.
It was a "nomadic" life of "turmoil" moving from Kyrgyzstan to Dagestan, before they settled in the Boston region in 2002.
The father, Anzor, fell sick. The mother, Zubeidat, never accepted by her husband's family, turned to fundamentalist religion, dressing in black and coaching Tamerlan.
After their parents, suffering from psychological problems, moved back to Russia in 2012, Tamerlan became the only adult reference for his little brother, Bruck said.