IF MANY AMERICANS were surprised by the indifference and apathy with which the Egyptian people responded to the assassination of President Anwar Sadat last November, David Hirst and Irene Beeson surely were not.

To them, he was not a brilliant statesman leading his nation to peace and glory but a fraud, pure and simple-- part buffoon, part mountebank, and all traitor to the Arab cause he professed to uphold. They believe the Egyptians saw through a facade that deceived Sadat's admirers in the West.

Hirst and Beeson argue that the completion of Israel's withdrawal from the Sinai is not a vindication of Sadat's policies, not a triumph of diplomacy over bloodshed, but the inevitable conclusion of a craven sellout of the "Arab nation" that can only cause more violence in the future.

In short, Sadat is pretty much the book one would have expected from Hirst and Beeson. They are respected British journalists with long experience in the Middle East who never made any secret of their antipathy to Sadat.

They were never caught up in the glamour of Sadat's television diplomacy. Hirst, in fact, was one of the few journalists to be thrown out of Egypt during the years when Sadat was courting the western press. He and Beeson never believed in Sadat's peace treaty or in the prosperity he said would follow it, and their book is a relentless indictment of his entire career. In their view, he was not just mistaken, but weak, corrupt and downright evil.

They add nothing new to the well-known details of Sadat's life and work, but they reinterpret them in the light of their harsh judgment. They debunk everything and spare nothing. Sadat's childhood, education, personal life and military career are all depicted as components of a corrupt personality.

He had, they say, "certain shallow gifts. He was an actor. He had the gambler's flair. But, above all, he was the consummate opportunist. . . . There was no deity that he would not dethrone, no principle that he would not abjure, no direction change, friend abandon, enemy embrace. He struck no attitude of which, in his real self, he was not the antithesis. He was constant only in his inconstancy. The politics changed, and out of recognition; only the person, his rhetoric, his very words--intemperate, demagogic, alternately abusive and obsequious--remained opportunistically, mendaciously the same."

Their style is vituperative and their case is strongly presented. But it gives Sadat's critics no new ammunition and it is unlikely to convince his admirers.

The many weaknesses of Sadat's character are already well-known. He was vain, grandiloquent, indolent and frequently deceitful. He was cruel to his first wife, he toadied to his superiors, and he rewrote his own life story to fit changing circumstances. He was an incompetent administrator, he made a mockery of the law in repressing his real or imagined opponents, and if he was not corrupt himself he fostered corruption among his intimates. All those blemishes are examined here in painful detail.

But it does not necessarily follow, as the authors would have us believe, that because of his flawed personality Sadat sold out the Arabs by accepting a premature cease-fire in the 1973 war. And even if one concedes, as Sadat never did, that the peace treaty with Israel is in fact a separate peace, Hirst and Beeson do not prove their accusation that he accepted less than the best peace terms available because of corrupt personal motives. If, as they say, "most Arab regimes" have fallen into "moral and political degeneracy," why then was it wrong for Sadat to free Egypt from their bankrupt policies?

In their attempt to prove that Sadat bungled the 1973 war and accepted a shameful cease-fire, they rely on an account by the Egyptian officer who was chief of staff at the time, Gen. Saad Shazly. They accept Shazly's charge that Sadat allowed the Israelis to cross to the west side of the Suez canal and trap the Egyptian Third Army through "a combination of madness, ignorance and treason." But they neglect to mention that Shazly's vantage point is that of a cashiered renegade in the pay of the Libyans.

History may yet validate Hirst and Beeson's assessment. But to accept their conclusions now it is necessary to believe that the Arabs ever had a realistic chance of dislodging Israel from the West Bank. It is necessary to believe that there is an "Arab nation," capable of unified, sustained policy and action. And it is necessary to believe that the Egypt Sadat inherited from Gamal Abdul Nasser--bankrupt, defeated, dependent on the Soviet Union, partially occupied by Israel--was better off than Egypt is today.