Richard Hornby at The Hudson Review:
Othello is clearly the protagonist of Shakespeare’s most problematic tragedy, yet it is Iago who incites the tragic action and pushes it through to conclusion. He has more lines than Othello, including more soliloquies. Othello’s murder of Desdemona is appalling, but his jealous motivation for it is clear enough; even though his jealousy has been triggered by Iago’s lies and innuendos, they do not mitigate Othello’s guilt, although they do raise the question as to why he is so gullible. (“This may be a lesson to Husbands, that before their Jealousie be Tragical, the proofs may be Mathematical,” wrote a late seventeenth-century wag.) It is Iago’s motivation that is troublesome. Even if he hates the Moor, why does he go to so much trouble to destroy him? Why not just kill him and be done with it? And why does Iago also have to wound Cassio, bring on the deaths of Roderigo and Desdemona, and kill his own wife? What have they ever done to him?
It is not that Iago gives no reasons for what he does. In fact, he gives several: he resents Othello promoting Cassio over him; he loves Desdemona himself; he fears that “the lusty Moor” (II.ii.292) has cuckolded him with his wife Emilia; he even fears that hapless Cassio has cuckolded him as well.