In both Mallory's Le Morte d'Arthur, and also in White's The Once and Future King, both author's tend to focus on King Arthur's downfall and the ruination of Camelot in much the same way. Both authors discuss the aspect of blind revenge as a major cause for Arthur's demise.
In both versions of the Arthurian Legend, both White and Mallory make it a point to discuss the fact that King Arthur turns a blind eye to the affair between his beloved Guinevere and his most trusted knight, Sir Lancelot. Arthur believes so much so in the concept of chivalry, which is most likely why King Arthur does not want to accept the tragic love affair, that it is in fact his downfall. His final recognition of the affair between his wife and his beloved knight lead Arthur down a path of revenge, from which there is no return. Arthur banishes Lancelot to France as punishment, but cannot simply leave it at that. Arthur is driven by revenge, and returns to France to war with Lancelot. Meanwhile, back home in Camelot, Mordred takes over as king and claims Guinevere in Arthur's absence.
King Arthur may have been able to save Camelot and his honor by not feuding with Lancelot after he banishes him. Both authors stress that it is Arthur's return to France that ultimately causes his downfall. He had already banished Lancelot, but could not overcome the great sense of betrayal done to him by his most trusted knight. It is King Arthur's obsession for revenge that is his ultimate undoing.
The whole kingdom of Camelot crumbles once one of the knights kills an adder with his sword, which glints in the sun, at once mistaken for a sign of war between Mordred and Arthur. Arthur is killed in both versions by Mordred in an act borne of unquenchable revenge. King Arthur is made human, more than legend, in both versions, as he cannot be the chivalrous king. Instead, Arthur makes the fatal mistake of being but a human man, thus unleashing vengeful thoughts and behaviors that lead to his downfall and the ruination of the beautiful and mystical Camelot.