Malory's Arthur is poignantly heroic. In Malory's shaping of the many stories of Arthur's adventures, we find a progression of character. Initially a young and eager king prone to conventional feats of military success, Arthur reflects his young kingdom and court. He draws the bravest knights to Camelot largely because of the opportunity he provides to use their strength for an idealized kingdom based on virtue. Rather than a more brutal view of government in which knights might feel more like mercenaries, Arthur exhorts them to bring law, order, and control to a wild land. His round table further speaks to an ideal of equal value among members in Arthur's kingdom.

Later adventures, particularly once Lancelot is included, are more psychological in quality, and both men in this love triangle reflect the internal as well as political divisions in the court. Lancelot and Guinevere, of course, love Arthur, as the most idealized and inspiring of kings. It is only right that they, who themselves are the best knight and the best queen, would want to serve the best king with the best vision. It's also only right that Arthur would love each of them as the highest form of the people he seeks to foster in his country. The impossibility of this triangle—the love and the betrayal—are of a piece and are critical to understanding the personal failings but the intellectual and political superiority of both men. That Arthur can inspire what seems like love in Lancelot, who is divided in his loyalties to his king and his mistress, suggests a quality in Arthur that is unusually three-dimension for a romance figure.

At the end, when Arthur has turned against Lancelot (not out of personal betrayal but out of political necessity), we again see the king lamenting the loss of the political ideal that defined his life. He himself has flaws, but he was capable of drawing to him the best of men and women. Only his own internal weakness (exemplified by his bastard son, Modred) is capable of defeating the external ideals he fostered.

If one applies Joseph Campbell's writings on the monomyth of the hero to Le Morte D'Arthur, we see that Arthur is indeed a heroic figure. Campbell's theory is called a "monomyth" because he found that the narrative elements and structure of many heroic myths across world cultures contained the same actions and themes. The legend of King Arthur and some of the corresponding characters in this story also lend themselves to this reading (such as the story of Percival and the Fisher King).

Arthur's story as portrayed by Malory is clearly easily broken down into Campbell's system (detailed in the attached document), beginning with the Call to Adventure: Arthur as a young boy in unaware of his noble birth, but feels the urge to free the sword Excalibur from the stone. When he is successful, it is understood that he must be destined to be king, according to Merlin's prophecy. Arthur is overwhelmed by the sudden attention and pressure on him, not to mention the fear of those who would wish to harm him (always a danger for a king), so his response is to run away into the forest (the Refusal of the Call). Merlin finds him, spends some time teaching him the ways of nature and some other lessons about honor, integrity and service, and Arthur finally agrees to become the boy king (Acceptance of the Call). The rest of the steps in the hero's myth are integral to the remainder of the story as well.

I think Arthur is heroic not only because he is brave and courageous but also because he strives to live up to the code of honor he sets for his knights.  He really tries to follow his code even when it is not of benefit to him.  For example, he give Mordred a place at court because his code of honor requires him to do so even when he know that Mordred is dangerous.  He does not want to acknowledge Launcelot and Guenever because it would break the code by bringing a lady to dishonor and would not be in the best interest of the kingdom.  Even after he is at war with Launcelot, he is willing to meet with him honorably.  Finally in the end it is not really Launcelot and Guenever that bring about his downfall, but it is the son he know he should not have trusted, Mordred, who brings about Athur's death.  There are several places when Arthur could have improved his fortunes by abandoning his code of honor, but heroically he did not.  Galahad may have been more pure, but he was not the rounded and complete character that Arthur was.

Arthur would have been considered particularly heroic by readers in Malory's time because although the chivalric code had been dying a long death since the Hundred Years War, it's ideals of  service to God, protecting the poor and defenceless, doing no dishonor to women, and treating enemies as well as friends with honor, were still prevalent ideals of society.  

A hero is the central character who possesses good qualities that enable him to triumph over someone who is evil. A tragic hero is a character who is dignified and noble, but he also has a tragic flaw that brings about his downfall. A cultural hero represents the values of his culture and provides a noble image that inspires mortals. Le Morte d'Arthur is not an epic in the traditional sense, but Arthur possesses the qualities of an epic hero.

Arthur is wise and strong, and his deeds determine the fate of his kingdom. He restores peace, but he has to fight against his enemies in order to keep his kingdom safe. He is courageous in his battles, reflecting the ideals of behavior in society. He establishes a code of behavior that requires his knights to be merciful, righteous, and honorable, values that Arthur feels are important. One of his greatest strengths is his ability to inspire great loyalty in his men. His knights will literally die for him. The Lady of the Lake and Merlin provide the magical elements of his life. Arthur is a larger-than-life figure, a source of national pride for the English, and a symbol of good fighting evil.

Today, the stories of Arthur still intrigue us, as adults and children. He truly is legendary.