The jury was expected to begin its deliberations later in the day after hearing from Tsarnaev's attorneys and the judge. Three people were killed and more than 260 injured when two bombs exploded near the marathon's finish line on April 15, 2013.
Tsarnaev, 21, was convicted by a federal jury last month of all 30 counts against him, including use of a weapon of mass destruction. The same jury must now decide his punishment.
"Merely killing the person," Mellin said, "isn't nearly as terrifying as shredding them apart."
The prosecutor showed a large photograph of 8-year-old Martin Richard, who was killed in the attack, and other children standing on a metal barricade near where Tsarnaev placed his bomb. He showed another photo of bloodied victims on the sidewalk.
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"This is what terrorism looks like," he said. Mellin said Tsarnaev showed no regret after the bombings, going to buy milk 20 minutes later.
"He acted like it was any other day," Mellin said. During the four-month trial, Tsarnaev's lawyers admitted he participated in the bombing, but told the jury he was "a good kid" who was led astray by his radicalized older brother Tamerlan, who wanted to punish the US for its actions in Muslim countries.