Ahab is destroyed well-before the Pequod meets Moby Dick. He is consumed by his over-riding fixation, his "mono-mania" directed at the white whale and characterized by irrational rage. This is true, of course, figuratively.
Before meeting the white whale, Ahab has already been overwhelmed by his obsession. His ideas and rage regarding the whale have become nearly as real as the whale itself.
“God help thee, old man, thy thoughts have created a creature in thee; and he whose intense thinking thus makes him a Prometheus; a vulture feeds upon that heart forever; that vulture the very creature he creates.”
Ultimately, Ahab is physically destroyed by this obsession as he refuses to relent in his pursuit of Moby Dick. He and his ship sink into the sea as a direct result of his ambition to throw down the god of the sea, the natural order it represents, etc.
This element of his fixation on the whale is not to be underestimated as it connects Ahab's outrage with his human condition - subject to natural limits and to a natural order. It is this set of unavoidable constraints that Ahab resists and that define his ambition.