The Richard III murder case is nearly 500 years old, and the verdict has long since been in: The king was guilty as charged - a lying, treacherous murderer.

No wonder. The public prosecutor was perhaps the most compelling case-maker of them all, William Sakespeare, whose indictment of the ill-starred British monarch (1452-1485) has played persuasively on the world's stages since about 1594. And continues to at the Folger Theater through Sunday night.

Did Richard get a raw deal at the hands of a fast-talking playwright who used his poetic license to libel and defame an innocent man?

Two thousand 20th century defense attorneys, calling themselves the Richard III Society, say yes - and are trying to prove it.

Richard III, in fact, is one of history's early victims of the Big Lie technique, says William Snyder, a retired Washington civil servant who is chairman of the society's 650-member American branch ("the livelist one," he claims proudly). He is also the author of a forthcoming book intended to vindicate Richard. "I'm great Shakespeare fan, but you do him an injustice to go to him in search of historic fact."

The society proudly notes its influence in the changed spiel of the Tower of London's warders; once they condemned Richard but now they adopt a more discreet tone. Other society accomplishments range from installation of an enameled shield marking the burial place of Richard's queen in Westminister Abbey to the fencing of the ground around King Richard's well, to the proselytizing of the media.

What attracts a 20th century American to the cause of a 15th century monarch? Snyder feels many new members are in some sense motivated by Watergate, and a consequent urge to ferret out conspiracies, even those in Tudor times.

William Hogarth, vice chairman of the society, says that even as a small boy he couldn't believe "such a cardboard villain was real" but since Shakespeare's play was so powerful, it served many as a "good goad for intense interest in the real Richard." In Hogarth's view, Richard's literary persecutor also doubles as his savior from the low estate of being one of history' more unsavory footnotes.

Shakespeare's opinion of Richard is clear: He was a liar, a cheat, a murderer who usurped the crown of England. To reach power, he killed an older brother and two innocent young nephews. He was also a rotten king for 2 1/2 years until the valiant army of the noble Henry Tudor (later King Henry VII) vanquished and killed him at Bosworth Field on August 22, 1485.

And, by the way, he was a hunchback, "deform'd, unfinisht," so ugly that dogs barked on him, "subtle, false and treacherous," and so mean that he spent two years in his mother's womb before being dragged out into the light of day - which, naturally, he hated. Surely a man fit only for history's deepest shadows. A Richard III Society? Why not a Genghis Khan fan club?

But do not be deceived by well-wrought phrases, counsels the society. Shakespeare's charges are easily debunked:

He was a horribly deformed "posionous . . . toad" who spent two years in his mother's womba.

The womb allegation is not taken seriously today. The deformity does not appear in pictures or sculptures made in his lifetime or while his memory was still alive. The picture in England's National Portrait Gallery shows no physical abonormalties, a face somewhat between plain and handsome and rather sensitive features.

He arranged the death of his brother, George, Duke of Clarence.

Actually, this brother was executed by the head of the family, King Edward IV, for treason. Clarence had, in fact, headed a rebellion against Edward, and Richard had loyally accompanied Edward into temporary exile.

He engineered the murder of the two young princes: "The most arch deed of piteous massacre that ever yet this land was guilty of."

It is (though uncertain) that the princes were murdered, though no one knows exactly where or when. Richard had little motive for murdering them, since he was already king when it happened (if not happened) and they had been formally declared illegitimate in the "titulus regius," a document prepared by Parliament which proclaimed Richard king. The Duke of Buckingham had more opportunity for such a murder. Henry Tudor, who had no solid claim beyond his military strength, had more motive. It is significant that Henry's first official act on taking the throne was to order all copies of the "titulus" destroyed. Only one was preserved and rediscovered much later.

In general, the tradition on which Shakespeare, drew for his play is seriously tainted with Tudor propaganda, the society insists. Shakespeare had the story from Holinshed, who took it from Hall, who got it from a fragmentary manuscript by Sir Thomas More that discussed only events in the early spring of 1483. The source of More's account (which he probably had not intended to publich) was his mentor John Morton, Bishop of Ely - who was, not conincidentally, one of those who engineered Henry Tudor's seizure of the throne.

Tracing the development of this history from one of Henry's henchmen to the poet who immortalized it. Snyder remarks, "It is interesting how Richard's depravity increases with the distances in time."

According to the available evidence, except for the shaky, partisan tradition on which Shakespeare's play was based, Richard was a decent man and a good monarch, who made reasonable, constructive laws. He was also, ultimately, a loser, and that was enough to darken his reputation because the winners write the history books.

"All of this does not reflect on Shakespeare," Snyder says. "I respect Shakespeare's dramatic integrity. He just saw a good story here and used it."

Snyder, a former examiner for the Bureau of the Budget and chairman of the American branch of the Richard III Society since 1971, does not give the impression of a crusader. Soft-spoken to the point of diffidence, he seldom misses an opportunity to point out that he is not a scholar and that there are others who probably are better qualified to unravel the legend surrounding the last Plantagenet king of England. But he has a law degree and sources by chapter and page, outlining the arguments in Richard's behalf, one wonders just what additional qualifications he seeks.

For the last seven years, Snyder has been working "on and off" on a 11-chapter book that explores the Richard myth. Now complete, it is the hands of his New York editor awaiting publication by the society.

Snyder's interest in Richard started in 1966 when he read Josephine Tey's "Daughter of Time," a 1951 detective novel that popularized the Ricardian cause. Soon after, he noticed the annual "In memoriam" run by the society in The New York Times and contacted the members.

"My wife got after me to do something near retirement, so I decided to attend the annual meeting of the Richard III Society in New York. We both went up and I didn't know what to expect. It was raining and the meeting was held in a pub. I thought maybe there would be handful of people, but when we went in there were about in 75 in the room."

That was October, 1966. By 1967, Snyder was chairman of the Washington branch of the society. In those days he held the meetings of the 10 to 15 area members in his house. Now the area chapter had grown to 40-plus members and they have to meet in public halls.

"Pursuing the Richard myth is a study in how people repeat a story and carry on tales. No one can ever say positively what happened and a mystery for which we do not have a final answer can be fascinating. I try to quote authorities and avoid idle speculation.

"The whole idea of the scientific mind is not to make judgments until all the facts are in and that is what I am trying to do."v