To what extent is he simply evil and searching for motives versus having valid reasons?To answer this question, look to a. his aside (164-174). b. his conversation with Rodgerigo (211-276). c. his soliloquy (278-304).

Iago is a complex character. Iago feels that he has been passed by for promotion in favour of Cassio. It would also appear that there are rumors that Othello has slept with Iago's wife, Emilia. We also must combine these ideas with a sense of deep-seated racism: Iago refers to Othello as a "Barbary horse," an "old black ram," an "erring barbarian." It is up to the reader to interpret how much of what Iago says relates to his true motives for going after Othello. In any case I think Shakespeare wanted to leave us with the idea that Iago is the Devil incarnate.