BOSTON, MA.—When a defendant stands trial for a crime that carries a possible penalty of death, what is the extra bar a jury must reach in order to decide that a lifetime of imprisonment is not enough? What damning facts must a prosecutor present, what heinousness of the crime and what painful details of the damage done to victims must be invoked to make the case that permanently removing an individual from free society is insufficient? What emotions, in other words, must be stirred in twelve individuals to convince them to unanimously agree to the killing of another human being?

At the Boston marathon bombing trial last week, as the penalty phase for Dzhokhar Tsarnaev began, on the witness stand for the government was a family friend of Lingzi Lu, a Boston University student killed in the bombing. She told the courtroom how Lingzi’s parents, after learning of her death, flew to Boston from their native China. Lingzi’s mother shopped at a mall for the right dress to bury her daughter in before heading instead to a bridal store. She settled on a flowing pink wedding gown that her 23-year-old daughter was laid to rest in.

Childhood images of another victim, Krystle Campbell, 29 years old, flashed on a video screen, showing her as a cherubic toddler in footie pajamas. Her father took the stand and told how he and his family initially were told by doctors that Krystle was injured but alive. Only later did they learn it was a case of mistaken identity and that she was one of the three fatalities at the bombing sites.

The stepfather of Sean Collier—the MIT officer who was shot point blank in the head by one of the Tsarnaev brothers, simply so they could get his service weapon as they attempted to make their escape—described Sean as wanting to be a police officer from a young age. Sean, he said, used to chase his little brother, Andy, around the house, making siren sounds. On road trips, every time the family saw someone pulled over by police, Sean would begin singing the Bad Boys theme song to the television show "Cops."

Jurors repeatedly saw the youngest victim, Martin Richard, eight years old, on a video recovered from the scene of the bombing. Prosecutors zoomed in on the spot where the little boy lay after the blast. Martin weakly raised his arms up just once as his mother, Denise, bent over him. Steven Woolfenden, who lost his leg in the blast, was with his then three-year-old son Leo, a few feet away from Martin, when the pressure cooker bomb exploded. He testified that after the explosion he heard Denise pleading over and over, “Martin, please. Martin, please.”

To sit through the penalty phase of a death penalty trial is to understand what a profoundly agonizing burden is placed on the jury. In ordinary criminal trials jurors determine a defendant’s guilt or innocence; it is the judge who hands down the sentence. Uniquely in cases where a defendant is found guilty of “death eligible” charges, the penalty for the crime is the jury's domain.

Though it has an empirical veneer, the process by which jurors reach their decision is surprisingly arbitrary. They must weigh the “aggravating” factors (circumstances that make the crime particularly heinous) the prosecution can prove against any “mitigating” factors (information or evidence that might lead to a sentence less than death) introduced by the defense. While the standards for what constitutes an aggravating or mitigating factor are fairly straightforward, the weight each individual juror gives to these factors, and how they arrive at the ultimate decision of life or death, is entirely personal and subjective.

In the courtroom where Dzhokhar Tsarnaev’s fate will be determined, Judge George O’Toole last week admonished jurors that they cannot base their decision on “sympathy for the victims.” Yet it is the particular calculus of the death penalty process that this is exactly the emotion that the prosecution needs to evoke—to humanize the victims to such a degree, to drive home to the jury the magnitude of their loss and suffering, to engender whatever level of hate for the defendant— is required for the jury to call for nothing less than his death.

There is a profound psychic cost to the litany of deeply painful, graphic, heartbreaking, almost numbing, images and scenes that prosecutors presented across three days last week. In addition to testimony from the friends and loved ones of those who died, multiple survivors, many walking into the courtroom on prosthetic limbs, detailed their terror during the attacks. We heard graphic accounts of the tearing of flesh and breaking of bones, and accounts of long recoveries, with terms like “limb salvage” frequently mentioned, and x-ray images of bodies still riddled with ball bearings and nails from the pressure cooker bombs. One man still has a BB lodged in his brain. Another man, already an amputee on one leg, was unsure whether he may yet lose his other leg. Yet another victim testified her husband is currently in a mental hospital as a consequence of the attack that injured them both.

Throughout this testimony jurors occasionally wept, as did many in the courtroom, including at least one court security officer and some of the journalists present. It is no surprise that many jurors sitting in capital murder cases are so traumatized by this process that a startlingly high number seek counseling after rendering their verdict. 

In a sense, the most difficult part of this decision remains ahead of the jury. Today, as Tsarnaev's lawyers begin to make their case for sparing his life, they, too, are invoking as powerful a narrative as they can of his life, hoping to inspire mercy in the jury. They have already begun talking about Tsarnaev’s childhood, particularly his relationship with his radicalized older brother, Tamerlan, who the defense says masterminded the attack. In contrast with the horrific nature of the crimes Tsarnaev has been convicted of, jurors will hear days, possibly weeks, of testimony on the young man's life: Before the bombings, he was by all accounts largely unremarkable, an amiable pothead with a wide circle of friends, a promising student with no history of violence.

The attorneys will have to defend Tsarnaev not because his actions are defensible, but because it his right to have a defense, and because they, like so many others, oppose capital punishment. Among western industrialized nations, the United States is the sole country that has the death penalty. Many reasons have been offered for its abolition: that it is immoral, that it is not an effective deterrent, that it is more costly than incarceration. It has been described as “a violent public spectacle of official homicide, and one that endorses killing to solve social problems.”

The emotional toll of revisiting the bombings in excruciating detail goes beyond the jurors or those in the courtroom. The family of Martin Richard includes his sister, Jane, who lost a leg in the bombing; his mother, Denise, who lost an eye; and his father, Bill, whose legs caught shrapnel and whose eardrums were perforated in the blast. Bill Richard took the stand during the earlier phase of the trial where Tsarnaev’s guilt was established, but the family has been notably absent from the penalty phase. Their position was made clear in an open letter to the Justice Department they authored and which was published this month on the front page of the Boston Globe. They asked the government, now that Tsarnaev has been found guilty of the bombing, to take the death penalty off the table. They wrote not expressly in opposition to the death penalty itself, but of a desire to move on and gain some measure of closure. Sentencing Tsarnaev to death would mean years of appeals that would mean their children would “grow up with the lingering, painful reminder” of what they had lost. “As long as the defendant is in the spotlight, we have no choice but to live a story told on his terms, not ours,” they wrote.

As I walked to the courthouse last week I noticed inscribed in the stone of the building wall a quote from former Supreme Court Justice Felix Frankfurter: “The responsibility of those who exercise power in a democratic government is not to reflect inflamed public feeling but to help form its understanding.”

If the goal was simply to put Tsarnaev away for life, then the spectacle of this trial was unnecessary. Tsarnaev’s defense team reportedly offered to plead their client to life imprisonment, but the government has insisted on pursuing the death penalty. In doing so they must necessarily pursue a path that keeps the pain of the victims alive in order to incite the most retributive emotions of the jury and the public.