The fall of Camelot in Le Morte d'Arthur was caused by many factors that led to its destruction, but the largest singular issue is discord among the knights of the Round Table. Even though they swore an oath not to fight one another, their inner feuding and squabbling perfectly illustrate the notion that a kingdom divided against itself cannot stand.
Then there is the affair between Gwenyvere and Launcelot. The knights at the Round Table who were jealous of Launcelot and did not like him to begin with use Launcelot's affair to stir up trouble for him in court with Arthur. Mordred and Aggravayne accuse him of treason, and factions form among the knights. While we are on the subject of Mordred, let it be said that his evil scheming for the throne also greatly contributed to the fall of Camelot. He instigates the feud between the knights' over Launcelot's treasonous affair with the Queen and then while Launcelot and Arthur and many of the knights are fighting over it in France, Mordred steals the throne and forges fake letters claiming that Arthur is dead. His evil maneuverings definitely contributed to the destruction of the Round Table and the fall of Arthur, since Mordred kills him in battle.
Mallory's recounting of the Arthur story is quite complicated, but I think the things that destroy Camelot are pretty simple—lust, jealousy, and vengeance—and of these, the greatest is lust. Sex and, in particular, illicit sex forms the basis for the story and marks all the important plot points. At the very outset, Arthur is fathered by Uther when he comes to Igraine's bed magically disguised as her husband. This sort of trickery is mirrored by Arthur himself, who has an illicit affair with Morgause, one of Igraine's daughters (and Arthur's half sister!). Morgause, as a result, gives birth to Arthur's nemesis Mordred, who eventually kills Arthur in battle. Guinevere's affair with Lancelot is another example of the destructive power of lust since the affair, and Arthur's discovery of it, leads directly to the bloody battle that decimates the Round Table and in which Arthur is killed. It is clear that Lancelot's (initially) chaste love for Guinevere is meant to represent the chivalrous ideals of the Round Table, just as their inevitable succumbing to temptation highlights the impossibility of living up to those ideals.