It is a work which offers high ideals about chivalry - ideals that seem to be at odds with the life story of its author.
Thomas Malory, a knight, was accused of a few armed robberies and kidnappings, but nothing came of the charges. Then, in 1451, an order was given for his arrest on charges of robbery, kidnapping, and the rape of a political rival's wife. Between then and his actual arrest, Malory and his associates continued to commit a number of robberies and a few kidnappings; there are over a hundred robberies of which they were accused. These robberies may have been political in nature, as the victims were mostly supporters of the Duke of Buckingham; it's also been suggested that some or even all of the accusations were politcally-motivated falsehoods.
Later that year, Malory stood trial and was jailed for his crimes, but released on the grounds that the jury had not been of his own countrymen. While there was never a retrial, he was arrested again and imprisoned shortly after his release on fresh robbery charges; he escaped. He later escaped again while awaiting trial on yet more charges of robbery and horse theft, and was never actually brought to trial.
During the Wars of the Roses, Mallory was a staunch Yorkist, and when the York claimant Edward IV took the throne, Malory was pardoned for these misdeeds. By 1468, however, he joined a conspiracy to overthrow the King. He was charged with treason, convicted, and imprisoned at Newgate; over the years two general pardons were issued to all those involved with the conspiracy, but both explicitly pardoned everyone except Sir Thomas Malory.
It is during his imprisonment for treason that Malory wrote his compilation of stories, drawn mostly from the French tradition. The knights of the Round Table in his book exemplify the virtues of what a knight should be; they are not bandits or brigands, and they serve their king faithfully (except for Lancelot and his illicit dalliances with the Queen) despite Arthur's own weaknesses (such as his incestuous relationship with Morgan le Fay). In short, they are paragons of all the attributes their author lacked: where Sir Galahad is perfectly chaste, Sir Thomas Malory was a rapist; where Sir Lancelot defends the weak, Sir Thomas Malory robbed those he could; where Sir Gawain put his own life on the line for his king time after time, Sir Thomas Malory plotted his own liege lord's overthrow. Whether the project was inspired by regret, or whether he never saw the contradiction of his work against his own life, Le Morte Darthur remains an essential work on the late medieval view of knightly virtue.