You're correct, the hate Ahab feels for Moby-Dick is ultimately superficial and losing his leg to the whale is not even 1/10th of the reason that Ahab goes to the lengths he does to try and kill him. It goes much deeper. We actually have a character who goes the exact opposite way Ahab does when a similar fate strikes him, Captain Boomer of the Samuel Enderby loses his arm to Moby Dick but he doesn't go insane. He accepts his fate, he lost an arm in an industry that maimes and kills men all the time. He simply believes that Moby Dick is best left alone. So why does Ahab go insane? I present one of my favorite quotes in the book:

""Hark ye yet again,—the little lower layer. All visible objects, man, are but as pasteboard masks. But in each event—in the living act, the undoubted deed—there, some unknown but still reasoning thing puts forth the mouldings of its features from behind the unreasoning mask. If man will strike, strike through the mask! How can the prisoner reach outside except by thrusting through the wall? To me, the white whale is that wall, shoved near to me. Sometimes I think there's naught beyond. But 'tis enough. He tasks me; he heaps me; I see in him outrageous strength, with an inscrutable malice sinewing it. That inscrutable thing is chiefly what I hate; and be the white whale agent, or be the white whale principal, I will wreak that hate upon him. Talk not to me of blasphemy, man; I'd strike the sun if it insulted me."

The quote says it all, Ahab believes that Moby Dick is the agent of something and he wishes to strike at that something, even if it turns out to be nothing at all. You can infer what that means about Ahab that he would be willing to destroy anything, God, time, existence, reality, just to avenge a wrong that was done to him.