It's sort of a twisted take on the traditional hero myth. Just read any Greek myth for example, the "protagonist" usually a god, was portrayed as being indescribably beautiful while the antagonist was earthly and bestial. The god usually journeys into the lair of the wicked monster, slays him, and the balance is restored. Alternatively, the monster rebels against god and is then righteously cast down by the latter. The manga takes this trope and turns it on its head where the roles played by the traditional hero and villain in a religious legend are reversed.
Also I think his design changes to emphasize Griffith's transcendence from human into apostle. Notice how his eyes are different, black orbs with white (yellow?) irises like those of a predatory animal.