In Le Morte D'Arthur, a desire for vengeance brings down the Round Table. It's about as simple as that. First, there's the blood feud that began early in the story, with Pellynore's slaying of King Lot. Then there's the grudge that emerges when Gawain's brothers are accidentally slain by Launcelot. Add that to the fact that on their many adventures these knights are often tasked with avenging who-knows-who's death, and you've got a pretty vengeful Camelot. But does all this vengeance get anybody anywhere? Not really. In fact, The futility and destructiveness of vengeance is undeniable, even when it is undertaken honorably. It usually ends badly for just about everyone involved, which begs the question – why are all these knights so vengeful in the first place?
The fact that it contributes to the fall of the Round Table shows us that vengeance is very destructive and totally pointless.
Gawain's reconciliation with Sir Launcelot is too little, too late. He has already gone too far down the path of blood feuds and vengeful violence, and there's nothing he can do to make it right.