In the last book of Le Morte D'Arthur, the whole Round Table comes crashing down around Arthur's ears. Some of Arthur's knights, who also happen to be his blood relations, really have it in for Sir Launcelot. They use Launcelot's continued affair with Gwenyvere to take the guy down, causing civil war to break out among the knights. This turn of events is especially sad because the Knights of the Round Table have an unspoken agreement specifically to not fight with each other.
On the other hand, it's not completely unexpected: hints of the discord brewing among Arthur's knights, and between Arthur and his vassals, are present right from the very beginning of Arthur's reign. Soon after he becomes king, Arthur's uncle, King Lot, leads an alliance of kings that challenge his lordship. And very soon after Lot's sons come to court, another one of Arthur's knights, King Pellynore, kills King Lot. Unable to let it go, Lot's sons kill Pellynore's son, Sir Lamerok. (You with us? That's a lot of killing.)
Arthur's knights take sides in the feud, which means that even before Arthur's nephew and son accuse Launcelot of treason with the Queen, factions have been formed. The line in the sand has already been drawn. Alas, it seems the unity of the Round Table was doomed from the very beginning.
The discord and blood feuding and hatred among the Knights of the Round Table are all very depressing. And, with this, the story could be trying to make the point that a kingdom divided against itself will inevitably fall, or even that a perfectly united kingdom can never exist in the first place.
On the other hand, though, the very end of the book shows Gawain giving up his enmity toward Launcelot on his deathbed in a letter in which he asks him to pray on his tomb. It also shows Launcelot and all his knights following Gwenyvere's lead into the religious life, and Launcelot being received in heaven by choirs of angels. This ending is not wholly unexpected, either: the book about the "Sankgreall," or "Holy Grail" showed Arthur's knights reaching for spiritual, rather than earthly, glory.
So, with this ending, the story suggests that maybe Galahad, the virginal knight whose spiritual prowess eventually achieved him eternal bliss, had the right idea after all. Unity between men on earth will necessarily be destroyed by our sinful pride; even the code of chivalry is not strong enough to withstand it. For that reason, why not pursue a higher goal – unity with God in heaven?