From Shelley, A Defence of Poetry (1821):
Nothing can exceed the energy and magnificence of the character of Satan as expressed in “Paradise Lost.” It is a mistake to suppose that he could ever have been intended for the popular personification of evil. . . . Milton's Devil as a moral being is as far superior to his God, as one who perseveres in some purpose which he has conceived to be excellent in spite of adversity and torture, is to one who in the cold security of undoubted triumph inflicts the most horrible revenge upon his enemy, not from any mistaken notion of inducing him to repent of a perseverance in enmity, but with the alleged design of exasperating him to deserve new torments. Milton has so far violated the popular creed (if this shall be judged to be a violation) as to have alleged no superiority of moral virtue to his God over his Devil. And this bold neglect of a direct moral purpose is the most decisive proof of the supremacy of Milton's genius.
(Illustration is from Gustave Doré, Paradise Lost, 1866.)