There is, as far as I have been able to find, one critical book based on The Once and Future King. It is (appropriately titled) T.H. White's The Once and Future King by Elisabeth Brewer. Since I am writing my thesis on White's book, I am also using Brewer's critical work. It has been quite helpful in a number of areas.
But sometimes, she is just so. very. wrong. It drives me nuts.
Brewer writes,
"While Mordred with his bitter contempt and hostility towards Arthur is a vile figure, he is nevertheless not unreasonably provoked, not only by the King's endless patience and forbearance, but also his lack of perception and his genial tactlessness" (112-3)
It is an ethical fallacy to assert that someone is provoked because another person acts with patience and forbearance. The person who claims, "He drove me to it by being too nice" has no valid basis for their crimes. Decency is no cause for enmity.
Furthermore, Arthur, as White writes him, devotes his entire life and rule to the promotion of an abstract Right; where good deeds are done not because one must but because one ought to. This is the standard he holds himself to, not retaliating with violence despite provocation. (After all, as King, he could have Mordred murdered for his accusations against Guenevere but does not because it would not be just). To suggest, that Mordred is at all justified in his course of action, especially on the basis of Arthur's behaviour toward him, is ridiculous. As White points out elsewhere, Mordred acts as he does for other reasons: his poisonous upbringing with Morgause, his physical deformity, the inheritance of an ancestral feud and in the end, his own insanity. The politeness of his King and father have nothing to do with it.