Legend has it that Camelot's ultimate demise was rooted in the queen Guinevere. That when she left, Arthur's spirit was crushed and he could not continue to be the man he once was.
But the legend was only half right.
His spirit was hurt when she left, but it wasn't completely defeated. Despite all that had happened, one thing had not changed.
Arthur still had Merlin.
But two weeks later, when Merlin was killed…
He was alone again.
And at that point, he simply could not force himself to go on, no matter who urged him. And thus, Camelot fell.