Every decision they make will eventually lead to them sealing their own fate. Looking at the scene where King Arthur has a dream about his own death, this becomes more clear.
After the dream Gawayne tells Arthur that if he does not make a truce he and many of his knights will actually die. Arthur decides that Gawayne is right and chooses to make a truce with Mordred. It seems, however, that fate is already predetermined.
Even though Arthur and Mordred make a truce Arthur still dies after he draws his sword to kill a snake. The question one might ask here is whether the snake is a coincidence or an act of fate.
And when King Arthur should depart, he warned al his host that, and they, see any sword drawn, “Look ye come on fiercely and slay that traiter Sir Mordred for I in no wise trust him”. In like wise Sir Mordred warned his host that “And ye see any manner of sword drawn look that ye come on fiercely, and slay all that hath ever before you standeth, for in no wise I will trust for this treatise.”
There's clearly no trust between the two parties involved and an enormous amount of tension. The second a snake appears Arthur draws his sword in reflex, and by doing so he seals his own fate.
He may have decided not to go to war and to survive, but he can't escape his destiny. The sneaky snake of fate comes in and starts the war anyway.
According to Marilyn Corrie this is a clear example of free will at work. She claims that King Arthur could have prevented the war and his own death by not drawing his sword:
Before relating the episode involving the snake, Malory has described how Arthur has warned "all hys oste" that they should "com on fyersely" if they see "ony swerde drawyn." Mordred has told his army exactly the same thing—"for in no wyse," he says, "I woll nat truste for thys tretyse" The knight who draws his sword to strike the snake, then, really