The Iliad Fate and Free Will Quotes

How we cite our quotes: Citations follow this format: (Book.Line). We used Richmond Lattimore's translation.

Quote #10

(Zeus:)
For if we leave Achilleus alone to fight with the Trojans
they will not even for a little hold off swift-footed Peleion.
For even before now they would tremble whenever they saw him,
and now, when his heart is grieved and angered for his companion's
death, I fear against destiny he may storm their fortress. (20.26-30)

Is it reasonable for Zeus to fear that Achilleus might act against his own destiny? Do you think the rest of the Iliad supports or contradicts this view of human freedom? If it contradicts it, does Zeus know something the rest of us don't know? Or is there just something special about Achilleus?