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.