How does Melville use Ahab and his mission to explore the mystery of evil?

The book's plot is built on one basic conflict: Ahab vs.
the whale. Ahab loses a leg to Moby-Dick; here is the wound that can never be healed.
The revenge scenario that follows is worthy of Shakespeare in its reach. Body and soul
blend together, are inseparable, in Ahab's mind. Ahab blasts Moby-Dick as incarnation of
evil and regards his task as that of a face-to-face encounter with evil. Is such a
mission sacrilege? Men such as Starbuck pointedly ask this question in the text itself.
Ahab suggests that the world is a fraud, an illusion. Ahab as Prometheus conducts a war
with God; he is staking out the limits of human doing and human reach. Have we
encountered an overreacher? "Striking through the mask" is Ahab's formulation, and the
theatrical metaphor conveys Melville's sense of living among shadows and illusory
surfaces. The corollary is that we can know reality only through an act of agency and
violence. Melville gaudily decks out Ahab in satanic colors. Melville proposes woe as
the eternal core of the soul. We discover madness also, as if the "dig" itself toward
the kingdom below were the very pulse of madness. Ahab presents an archaeological depth
of character. Melville is at his most theatrical when he suggests that great passion and
madness are a form of takeover, that the hostage self is eclipsed. Melville carries this
view of madness to its grisly conclusion in a view of marionette-like behavior: the
vacated self has no authority left, as it gesticulates and acts in its mania. The final
loss of self is shown in the black cabin-boy, Pip, who falls overboard and witnesses the
indifferent gods; he is "vacated" by the experience, and the book's most haunting
passages are related to his "orphaned" vision. We are made to understand that Pip is
precisely Ahab's alter ego, that the monomaniac and the witless idiot are versions of
each other. Ahab sees that Pip will be his own undoing. This is a ghostly kind of
fraternalism, different than the kind that binds Queequeg and
Ishmael.