The Metropolitan Opera in New York on Monday will present John Adams ’s opera “The Death of Klinghoffer. ” The organization’s decision to mount the production has already spurred protests, with more to come.

A too-brief summary: In 1985 Leon Klinghoffer, a 69-year-old disabled man, and his wife, Marilyn, were passengers on an Italian cruise ship, the Achille Lauro. The ship was hijacked by Palestinian terrorists, who shot Klinghoffer in the head and threw him overboard in his wheelchair.

John Adams is a serious artist, recognized as a leading creator of modern operas. “The Death of Klinghoffer,” first produced in 1991, contains a running debate between the killers—who voice a number of undisguisedly anti-Semitic slurs in the course of justifying their conduct—and their victim. Protesters are demanding that the opera be canceled; defenders couch their position, as has the New York Times , in terms of artistic freedom or—as one letter-writer to the Times put it—of helping us “understand the anger, frustration and grievances of other people.”

So, in Joan Rivers ’s much repeated phrase, can we talk? Some things are easy. Mr. Adams’s opera is protected by the First Amendment and so is the Metropolitan Opera in its decision to offer it. It would be a gross and obvious constitutional violation if government sought to bar the opera from being publicly produced or imposed any punishment for doing so.

Beyond that, canceling any public artistic performance because it expresses unpopular or even outrageous views is dangerous. I represented the Brooklyn Museum when then-New York City Mayor Rudolph Giuliani in 1999 sought to shut it down because he viewed some of its art—I use his language now—as “sick,” “disgusting” and sacrilegious. I argued then, successfully, that the mayor’s conduct violated the First Amendment.

But the controversy over the Adams opera cannot be dealt with by simple reference to the First Amendment or artistic freedom. Those who direct the Metropolitan Opera made a choice when they decided to offer Mr. Adams’s opera, and it is altogether fitting that they be publicly judged by that choice.

Suppose the opera had been about a different murder and the Met offered an intense, two-sided operatic discussion of the desirability of the murder of, say, President Kennedy in a work called “The Death of JFK. ” Or a production about the murder of Dr. Martin Luther King Jr. in which singers on the “side” of that assassination offer racist views in support of the murder. Or how about one on the death of one of the thousands of victims of the 9/11 attack that contained an extended operatic debate between her killers and herself about whether her death was justified.

Surely we recoil at all of these. They all would be protected by the First Amendment. The First Amendment is basically—and gloriously—content-neutral. It protects not only enduring works of art but also the dregs of human imagination, ranging from films of animals being tortured and killed to the publication of “Mein Kampf.” But it is inconceivable that the Metropolitan Opera would have chosen to offer the public any of the operas I have just hypothesized.

Why then offer one that equates—sympathetically, no less—the murderers of Leon Klinghoffer with their victim? “Grievances” there may be on both sides in the Middle East conflict, but there was no moral justification for the murder of Klinghoffer. John Adams has defended his focus on the motivation of the killers by saying that it helps to explain “what in the mythology that they grew up with, forced them or dared them to take this action.”

But the killers were not “forced” to murder Klinghoffer. Nor were they dared to do so. They chose to commit their crime. So did Lee Harvey Oswald, James Earl Ray and Osama bin Laden. We can expect no arias to be sung in their defense at the Metropolitan Opera, and there is no justification for any to be sung for the Klinghoffer killers.

Suppose the Oxford Union proposed a debate on the topic of Mr. Adams’s opera and it was phrased this way—“Resolved that the killing of Leon Klinghoffer was justified.” Suppose you were asked to take the negative side of that debate and to argue that he should not have been murdered. Would you do so? I hope not. I hope you would say that the subject is not one on which any rational, let alone morally justifiable, debate is possible. One can argue passionately about the Middle East, Israel or Palestinians, but nothing makes the Klinghoffer murder morally tolerable.

The great scholar Alexander Bickel recalled in “The Morality of Consent” (1975) that he had heard that in the tumultuous late 1960s a crowd had gathered outside an ROTC building at a great university, where members of the faculty joined students discussing “the question whether or not to set fire to the building.” The faculty members, Bickel surmised, took the negative, the matter was ultimately voted on, and the affirmative side narrowly won. Bickel’s conclusion: The “negative taken by the faculty was only one side of a debate which the faculty rendered legitimate by engaging in it. Where nothing is unspeakable, nothing is undoable.”

That’s where I come out on the Met’s decision to offer this opera. What Prof. Bickel wrote applies here: Where nothing is unspeakable, nothing is undoable.Leon Klinghoffer’s murder was an unspeakable act. Period. His demise is not a proper subject of debate, only of mourning. And of how best to prevent future murderous attacks.