What did Moby Dick represent to Ahab?

Throughout the novel, Melville creates a relationship between Ahab and Moby Dick despite the latter's absence until the final three chapters through the recurrence of elements creating a close relationship between Ahab and the whale. The most significant of these is the actual physical presence of the Sperm Whale as part of Ahab's body in the form of Ahab's ivory leg. The whale is a physical part of Ahab in this instance; it is literally a part of Ahab. Melville also develops this theme through the uncanny sense that Ahab has for the whale. Ahab has a nearly psychic sense of Moby Dick's presence, and more tragically, the idea of Moby Dick perpetually haunts the formidable captain. This theme serves in part to better explain the depth of emotion behind Ahab's quest for the whale; as a living presence that haunts Ahab's life, he feels that he must continue on his quest no matter the cost.

Is Moby Dick evil to Ahab?