The workings of fate versus free will are all over the place in The Once and Future King. From King Pellinore—who Arthur notes wouldn't even hurt a fly—accidentally killing King Lot, to Arthur not knowing his parentage and accidentally sleeping with his sister, almost every major character is influenced by an inescapable fate that seems impossible to overcome by exercising free will.
One of the more vivid metaphors for fate in the text is that it's "like a ball in a groove" (C.11.200). It's stuck, and can't move. Once it's decided, it will not deviate from its course, come what may. Fate also works on both the micro and macro levels here: it's the difference between minor character traits like Kay's sarcastic and saucy attitude, and an entire tragic arc such as Mordred's conception eventually leading to Arthur's death.
Questions About Fate and Free Will
How do fate and free will contribute to the tragedy of Arthur's story?
Where do we see characters exercising free will and overcoming things that appear to be predestined?
What is the relationship between Fate and Time in White's novels? How are they linked?
Where do you see characters undergoing sudden reversals of fortune, and what role do you think fate and free will play in this?
Chew on This
Merlyn could have stopped Arthur from sleeping with Morgause if he had not been so wrapped up in hurrying up and getting to his meeting with Nimue. So in a way the entire tragedy is Merlyn's fault.
While the outcome of Arthur's life might be pre-determined, the process by which he gets there is not. He has plenty of room to exercise free will along the way.