The character of Iago in Shakespeare's Othello has been much debated. The most common view is that since he is jealous of Othello and hates him, Iago plots revenge. And yet it is not necessarily clear that his own statements are meant to be taken at face value.
As a villain, Iago has several precursors in English literature. In particular, he appears to derive from the archetypical Vice of early Elizabethan morality plays. What sets him apart, however, are the endowments of genuine character that he displays--including charm, wit, and especially reasonable motives. In this last regard, Iago is unique, for the Vice of the religious plays was an abstract personification of evil and disorder who delighted in mayhem for its own sake.
In drawing Iago, however, Shakespeare is careful to give him at least two plausible motives for his malevolence. Indeed, we have scarcely met him before he confides in us his reasoning for seeking the downfall of the Moor. First, he claims to be incensed that his general, Othello, has chosen a less qualified and less proven subordinate to be his lieutenant (I:i:8-33). He complains that "three great ones of the city" have personally intervened on his behalf, only to be told that Michael Cassio has been chosen instead--someone he says is much less entitled to the position. Consequently, "I follow him to serve my turn upon him . . . " (I.1.42).
If the appointment had not been made by "letter and affection" Iago implies, but by "old gradation, where each second stood heir to the first," then he and not Cassio would have been promoted. "The implication is that Cassio was junior to Iago either in years or in service, or in both," observes Richard Flatter (34). "Yet we never gain the impression that Cassio is younger than Iago." Noting that it was Cassio who went with Othello during the wooing of Desdemona (III.3.71), Cassio is more likely to be close to Othello in age than to be a younger man...