The penalty phase of Dzhokhar Tsarnaev’s trial is underway, and federal prosecutors have been getting some well-publicized advice about the penalty they should seek for the Boston Marathon terrorist.

In a statement featured on the Boston Globe’s front page last week, the parents of 8-year-old Martin Richard — the youngest victim murdered by the Tsarnaev brothers — said they would be in favor of the Justice Department “taking the death penalty off the table” in exchange for a life sentence and the waiver of any right of appeal. On Monday, the Globe spotlighted a similar call by Jessica Kensky and Patrick Downes, newlyweds who both lost legs in the 2013 bombing.

To their credit — and in keeping with the grace and decency they have shown from the beginning of this terrible ordeal — Bill and Denise Richard emphasize that they speak only for themselves. Kensky and Downes likewise acknowledge that their views are theirs alone, and “promise to continue to listen thoughtfully to opposing views as this public discourse continues.” Prosecutors, for their part, have responded with compassion and courtesy. US Attorney Carmen Ortiz said she cares deeply about the views of the Richards, just as she does about those of other survivors and victims she has heard from, on all sides of the issue.

But the prosecutors’ job is not to carry out the wishes of victims and their families. It is to bring the murderer to justice. And in our legal system, justice requires a fair trial, an impartial judge, and a jury to weigh the evidence and come to a considered verdict — in short, due process of law. The desire to let a criminal’s fate be decided by those he harmed most directly can be overwhelming. But “justice” without due process is perilous. Leave punishment in the hands of victims and their kin, and the results are often blood feuds and revenge killings and vigilante violence.

The prosecution of the marathon bomber is styled “United States v. Dzhokhar Tsarnaev” precisely to make clear that it is the interests of the public that are to be vindicated. The crimes committed by Tsarnaev were horrific. The suffering he inflicted on so many innocent human beings was unspeakable. No one with a heart begrudges the survivors their right to express an opinion — any opinion — about the penalty the guilty man should be made to pay.

But in a civilized society, that penalty must be fashioned by the public, not by victims. The ongoing anguish of survivors may bring us to tears. We may be stirred beyond words by the dignity with which they bear their losses. We may yearn for them to be granted whatever they think will bring them closure and peace of mind.

Nonetheless, the survivors don’t get a vote. Only the jurors do — jurors empaneled in the first place only after elaborate scrutiny and questioning to be sure they aren’t biased. It goes to the very essence of due process that victims, or anyone with a personal connection to a case, not be permitted to render a verdict or to determine how guilt should be punished.

Inevitably, the prosecution of Tsarnaev has been tangled with the never-ending debate about capital punishment. All the familiar compass points of that debate have been represented. There are those who think that execution is the only just response to a massacre so cruel and heinous; those who are convinced that life in prison would be an even more excruciating fate; those who believe that the death penalty is never justified, regardless of the crime. Some argue that letting Tsarnaev live would amount to an ongoing mockery of his victims. Others claim that putting him to death would reward him with the “martyrdom” he craves. Everyone is entitled to a point of view, no matter how emotional or irrational or whimsical. In the court of public opinion no attitude is out of bounds, and there are no rules to determine which arguments prevail.

But in the federal courthouse on the South Boston waterfront where Tsarnaev’s destiny is being decided, there are rules aplenty. Each one is meant to ensure a verdict grounded not in rage or revenge, but in fairness and integrity. Tsarnaev will not be sentenced to death unless 12 jurors unanimously agree that that is what justice requires. He may deserve no more than the wanton brutality he showed his victims. What he will get instead is due process of law.

Jeff Jacoby is a columnist for The Boston Globe.