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Death penalty protesters gathered on Wednesday outside the John J Moakley Federal Courthouse during closing arguments in the sentencing phase of Dzhokhar Tsarnaev's trial in Boston. CreditCj Gunther/European Pressphoto Agency 
BOSTON — After 10 weeks of testimony and more than 150 witnesses, the jury of seven women and five men began the task Wednesday of deciding whether to sentence Dzhokhar Tsarnaev, the convicted Boston Marathonbomber, to life in prison without parole or to death.
Before the jurors began deliberations, they heard an impassioned argument from the prosecution that Mr. Tsarnaev, 21, should be sentenced to death because he was a coldblooded, unrepentant killer who had terrorized Boston and, in turn, the entire country by bombing innocent people, including children.
“This is what terrorism looks like,” Steven Mellin, a prosecutor, told the jury in his hourlong closing. “It’s Martin Richard bleeding on the ground in agony while his mother bends over him and begs him to stay alive, saying, ‘Please, Martin,’ ” Mr. Mellin said, referring to the 8-year-old boy killed by a bomb planted by Mr. Tsarnaev. The jurors, who are not sequestered, heard an equally impassioned argument from the defense. Judy Clarke, the lead defense lawyer, told the jury in her 75-minute closing that while Mr. Tsarnaev’s crimes were “senseless and catastrophic,” he, himself, was not the “worst of the worst,” for whom the death penalty is reserved.
“We ask you to choose life, yes, even for the Boston Marathon bomber,” Ms. Clarke said. She added that a sentence of life does not dishonor the victims or minimize Mr. Tsarnaev’s crimes. “The sentence of life allows for hope, for redemption, for healing for everyone involved,” she said. “It’s a sentence that reflects justice and mercy.”
The jury mulling Mr. Tsarnaev’s fate is the same one that convicted him last month of all 30 charges against him in connection with the 2013 bombings, which killed three people, maimed 17 and wounded more than 240 others. He was also convicted of killing an M.I.T. police officer and of participating in a carjacking and a violent shootout with the police.
The closing arguments on Wednesday covered familiar terrain. The lawyers disputed the degree to which Tamerlan Tsarnaev, the defendant’s older brother, had brainwashed Dzhokhar into joining in the bombings. And they offered sharply divergent views on how bad a life sentence would be inside the supermax prison in Florence, Colo.
But they also ventured onto new ground, particularly as they debated the value of testimony from Sister Helen Prejean, a nationally known opponent of the death penalty. She testified Monday that she had met with Mr. Tsarnaev five times and that “he was genuinely sorry for what he did.” She quoted him as saying of his victims, “No one deserves to suffer like they did.”
Ms. Clarke told the jury that the young man who spoke respectfully with Sister Prejean was not the angry, vengeful, unchanged, unrepentant jihadi portrayed by the government.
“The government will argue that this young man pulled the wool over this nun’s eyes,” Ms. Clarke said, but “she’s not going to come in here and lie to you.” Sister Prejean’s testimony, she said, shows that Mr. Tsarnaev is “on the path of growth and remorse” and “shows the great potential for redemption.”
The prosecution was having none of it.
“Did her testimony really give you any insight into what the defendant truly believes?” asked William Weinreb, the government prosecutor who gave the rebuttal to Ms. Clarke’s closing. “As a nun, she undoubtedly tries to see the good in everyone.”
Mr. Weinreb said that what Mr. Tsarnaev told the nun was akin to what he wrote in the boat in which he was captured: “Now, I don’t like killing innocent people. It is forbidden in Islam. But due to” (a word here was obliterated by a bullet hole) “it is allowed.”
He argued that the writing expressed the views of a terrorist. “Now that he’s on trial for his life, saying no one should have to suffer like that doesn’t tell you much about his core beliefs.”
Mr. Weinreb also belittled the nun for her comment on Mr. Tsarnaev’s youthful appearance. “Sister Helen is 76, and the defendant is 21,” he noted. “Of course he seems young to her.”
Judge George A. O’Toole Jr., who is presiding in the case, spent 90 minutes Wednesday morning giving technical instructions to the jurors on how to arrive at their decision.
The primary question before them will be whether the government has proved beyond a reasonable doubt that Mr. Tsarnaev’s crimes were so heinous — crimes the judge defined as “shockingly atrocious” — that he deserves to be put to death.
But arriving at an answer is not a simple yes or no proposition.
Of the 30 charges of which the jury convicted Mr. Tsarnaev last month, 17 carry the death penalty. The jurors will be poring over a lengthy and complex verdict slip that asks them to decide whether the government has met the test for the so-called aggravating factors that would argue for the death penalty for each of the 17 charges. The aggravating factors include the depravity of the crime, the intent to kill children, the indifference to human suffering and the apparent lack of remorse.
At the same time, the jurors must consider whether the defense has met the test for the so-called mitigating factors that would argue for life in prison. These include Mr. Tsarnaev’s youth as well as the unstable home life created by his mentally ill father, emotionally volatile mother and violent older brother.
The idea behind such a detailed checklist is to keep the jurors focused on relevant, legal factors and to try to lessen the influence of emotional factors.
But the process of weighing factors “is by no means a mathematical or mechanical process,” the judge said. He told the jury to consider the significance of each and allow for a reasoned, moral response.
“No juror is ever required to impose a sentence of death,” he said.
Jurors must be unanimous in their decision to sentence Mr. Tsarnaev to death. Otherwise, the judge said, “I will impose a mandatory sentence of life in prison without the possibility of parole.”