Shortly after a Cuban tribunal sentenced Humberto Sori Marin to death, his mother went to visit Fidel Castro to plead for her sons's life.

Marin and Castro had fought as comrades in the mountains, and after the revloution they often dined togehter at Marin's home with Senora Marin doing the cooking. At the meeting, Castrol assured her: "Don't worry, nothing will heappen to Humberto." The next night, Castro himself ordered the execution.

That incident comes from the pen of Armando Valladares, whose book, "Against All Hope," is an account of the 22 years he spent in various Cuban prisons for the "crime" of speaking out against communism. To say that the book is compelling is to understate its power; to say that it is horrific is also an understatement. With this book, Fidel Castro takes his place as yet another of this century's mass murderers.

The execution of Sori Marin was just another day's work for Castro. Turning on enemies and former colleagues alike, the Cuban dictator dispatched several thousand political prisoners (the exact figure is unknown) and imprisoned countless others. Valladares gives an account of a Latin gulag where prisoners were terrorized, beaten, starved, tortured and casually executed, often on the caprice of some uniformed sadist. Many were like Valladares -- convicted by tribunals that, for the sake of efficiency, handed down their verdicts before the trial had begun.

One of the benefits of being a liberal in a conservative era is that easy assumptions get challenged. One of those assumptions has been that Fidel Castro was not, all in all, such a bad guy. He was credited with improving the standard of living -- particularly health care -- for most Cubans, with cleaning up notorious Havana (the prostitution capital of the Western Hemisphere) and, of course, with toppling the repressive Batista regime. It was conceded that he was a dictator, that he was responsible for human rights abuses. But it was argued that these were insignificant and paled in comparison to what was happening elsewhere on the hemisphere -- Chile, Argentina, Guatemala and El Salvador. Liberals held their fire.

In an essay in a recent issue of The New York Review of Books, Aryeh Neier tries to account for such a double standard. Neier, vice chairman of the Americas Watch Committee, attributes the left's preoccupation with atrocities by rightist regimes to the tendency of those regimes to label their own enemies Castroites. It seemed that to concede the case against Castro would also concede the case right-wing dictators were making against their own dissidents -- not to mention the case being made by American conservatives. The reasoning is no more sophisticated than the old maxim that the enemy of my enemy is my friend.

Neier's credentials as a critic of all oppressive regimes are beyond reproach. He offers some reasonable justifications for what amounted to Castro's playing American liberals for patsies, but they in no way take the left off the hook. In fact, not only were Castro's crimes ignored, but the man himself was depicted as the romantic revolutionary -- a baseball-playing companero, a macho Hemingway type in the land of "Papa" himself. Castro's compelling and attractive antics totally overshadowed the sinister aspects of his reign -- so much so that even conservatives, who loathed Castro for his communism, remained ignorant of the true nature of his regime.

For whatever reason, the American left is at last coming to terms with Castro. The Neier essay, plus the reviews that "Against All Hope" has received in The Post and The New York Times, have done much to rectify matters. Now it is conservatives who follow false messiahs. President Reagan's characterization of virtually any Third World anticommunist as a "freedom fighter" is the moral equivalent of calling Castro an agrarian reformer. We await patiently the mea culpas from the right.

According to Americas Watch, there remain at least 110 political prisoners in Cuban jails and hundreds more in so-called "political education programs." Some of them have been incarcerated for more than 25 years -- old men whose executions have effectively been played out in slow motion. In "Against All Hope," Valladares tells their story just as surely as he tells his own. Through the personal intercession of French President Francois Mitterrand, Valladares was freed from Castro's grasp. Through his book, so have we all been.