It partly depends on what you mean by an epic hero and what version of the Arthurian legend you are talking about.
Was King Richard an epic hero? Was Theodore Roosevelt an epic hero? Was King John of England an epic hero? Was Napoleon an epic hero? Is Doctor Who an epic hero? If not (for one or more of them), why not?
“The Betrayal of Arthur” is just another study by an author who cherry-picks from different versions of the legends those features that agree with his thesis. The phrase “committed incest with abandon” is a give-away. In only some of the medieval tales is Mordred said to be Arthur’s son by incest rather than his nephew. In the only three accounts (all different) that give any details of how Mordred was fathered, Arthur and his sister are partly excused because they are unaware of their relationship. How is this incest “with abandon”?
“For instance, were you aware that after Arthur had unwittingly conceived Mordred on his own sister, he then, Herod-like, ordered the murder of all male infants born nine months after the seduction? What king would do that? In the end it is no wonder his son eventually turns against him - Arthur tried to murder him first.”
I am well aware of this account from the “Post-Vulgate Cycle” (from which it is taken by Malory). I am also aware of contradictory tales in other romances. For example, in the “Vulgate Merlin” there is no attempt at murder and Mordred is brought up by his mother and supposed father in his own home.
Also, in the “Post-Vulgate Cycle”, Arthur is not trying to murder his son begotten on his sister. He is trying to kill all children born on May Day because Merlin has prophesied that Arthur's kingdom will be destroyed by a boy-child born in the kingdom on May Day but will reveal no more. Merlin has also prophesied that a son begotten by Arthur by his sister will cause him great harm, but Arthur seemingly makes no connection between the two prophecies otherwise he would have easily found the child.
I am also aware of the depiction of Arthur in the German romance “Wigalois” as a perfect human being. Both accounts are fictional and almost certainly neither is the “original Arthur'' who is unknown to us. I am aware, from other tales of MANY legendary kings who have done that including King Herod. A very stupid rhetorical question.
And Mordred is often shown to be a murderer and rapist in matters unrelated to Arthur. See http://kingarthur.wikia.com/wiki/Mordred . On the other hand, mentions in early Welsh literature display Mordred as famed for his courtesy.
Is Oedipus, who married his mother, an epic hero? Some would say so. I do. Heroes are very often flawed. Theseus of Athens raped Helen when she was a young girl. Greek epic kings and epic heroes often come to ignoble, tragic ends. Jason, Bellerophon, Agamemnon, Achilles, Neoptolemus.
“Arthur's early years as king was a time of great achievement-politically, culturally and socially.” Bullshit. The romances almost NEVER get into the matter of political, cultural, and social achievements, other than to generally, but not always, to portray Arthur’s reign as glorious. For example, to ask how or whether the system of justice was better or worse under Arthur is to get into matters which are never discussed in the tales (except for occasional implied criticisms of trial-by-combat).
“Almost everything most people have read about King Arthur is wrong.” The Arthur who has come down to us is fictional. Almost any point made by one tale can be contradicted by another. There is no “right” or “wrong” version, except that historically all late versions are most probably wrong. The author is also mostly adhering to the romantic legend that has evolved over time, in which Lancelot was added to the main tale of Arthur, and in which the incest motif was added to the tale. Versions in which King Arthur is evil become “right” for this writer and those which don’t mention the evils become “wrong”.
Epic kings like Conchobar and Charlemagne and King David are also guilty of great wrongs. Charlemagne was the true father of Roland in some versions, whom he fathered in incest on his sister. Hrólf Kraki, the great epic king of Denmark, was (in legend) the son of King Helgi who unwittingly married his own daughter. Julius Caesar, in legend, is the father of Brutus who killed him.
The same story motifs tend to become attached to legendary kings again and again.
As great epic kings are made the subject of legend, great wrongs also tend to be ascribed to them. That does not make them less “epic”, just more evil.
Arthur, even in late versions of his tale, is still an “epic” king who defeats Frollo, ruler of Gaul, in single combat and so himself becomes the ruler of Gaul.
The “Post-Vulgate Cycle”, which as far as we know first introduced the story in which Arthur attempted to kill the infant Mordred, was written by an author who delights in tales of feuds and kin-murder unknown to earlier accounts and has a rather amoral attitude. Erec kills his own sister, compelled by a rash boon. Gawain murders Driant and Lamorat (Lamerok) and lies about it. Arthur begets a son, Arthur the Little, by rape. King Mark is also father of an illegitimate son by his own niece whom he tries to kill.
Arthur is sexually loose in some tales and has various illegitimate sons. But in the “Perlesvaus” Arthur is totally faithful to Guenevere. Which version is “right”?
One might as well try to interpret the Gawain of “Gawain and the Green Knight” by looking at Gawain as a treacherous murderer in the “Post-Vulgate Arthurian Cycle”. But that doesn’t work. But a novelist might make it work by making the adventure of the Green Knight the beginning of Gawain's downfall.
Kay is some romances is a cowardly murderer. In others, though rather outspoken and overconfident, he is a good and honourable knight. Neither version is “true”.
It is attempts to combine discrepant versions of the character that are wrong. Mix versions to create your own if you are writing a novel. But to take the worst versions of Arthur from different versions of the legend and attempt to portray this as the one true version is dishonest. It is also dishonest to take features mainly from late versions of the Arthurian tales and claim that this version is in some sense the original Arthur.
In any case, even in the “Prose Lancelot'' and in the “Post-Vulgate Cycle”, despite adulteries and other crimes, Arthur is still treated as an epic king.
Similarly, in the “Nibelungenlied”, Hagen is a great criminal and the murderer of Siegfried. But he is also a great epic hero, like his counterpart Efnisien in the “Mabinogion”. Heracles murders his own children when he goes mad.
The faultless, stainless king of popular imagination and Tennyson is no more right or wrong than the Arthur of “Le Morte d’Arthur”. Both are fictional. It seems that Sara Douglass was teaching about this stainless Arthur, and only after years of teaching actually read the medieval texts. Now she pretends that almost everyone was as deceived as she was. She overreacts. And she lies. That the late tale of Arthur was supposedly a moral tragedy is well known. If she was teaching Arthur, she should have known that. What was she teaching?
Mark Twain knew that medieval nobles were ignorant and brutish and portrayed this in his “Connecticut Yankee in King Arthur’s Court”. Nothing new there.
Medieval Arthurian literature was literature for the knightly class and rather obviously pandered to their prejudices. Of course, by modern standards, Arthur was far from an enlightened king. High birth matters. Prowess matters. Learning is something that clerics do. Farmers, merchants, and serfs barely appear. They are just commoners.
In a long speech on knighthood it is the Lady of the Lake who explains to the young Lancelot that it is by right that the commoners owe obedience to the knightly nobility and to the Church. A cleric does blame Arthur for too much favouring the knightly class over commoners, but this passage in the “Prose Lancelot” is very unusual.
Accidentally and unknowingly fathering a son on your sister is not more of a severe moral crime than just fathering a son in adultery. In reality, in the texts as we have them, Mordred’s treachery is recorded first. Only in late texts is the tale found that Mordred was, in reality, Arthur’s son. Yet this is the version that appears in almost all modern novels. It is false to think this is little-known.
Morality is rather ad-hoc. In one passage it is explained that Lancelot's sin with Guenevere was destined because Lancelot’s father had lain, out of wedlock, with the daughter of the Lord of the Fens and had sired Hector of the Fens, Lancelot's illegitimate half-brother.