TALBOYS.
Coleridge calls Iago's "a motiveless malignity." He hated Othello for not promoting him, but Cassio. That seems to me the real, tangible motive—a haunting, goading, fretting preference—an affront—an insult—a curbing of power—wounding him where alone he is sensitive—in self-esteem and pride. See his contempt for Cassio as a book-warrior—and "for a fair life"—simply like our notion of a "milksop." Why Othello, who so prizes him for his honesty as to call him ever "honest Iago," keeps him down, I have not a guess—