Ah, the green-eyed monster!
Iago is mad (at his boss Othello) that he has been passed over for a promotion (in favor of Cassio) to a position he feels he was more qualified for (he makes a strong case) vows to destroy Othello. Iago is also mad because he thinks his wife, Emilia, has slept with Othello, so he vows to match him "wife for wife" or create a situation that ruins Othello and Desdemona's marriage.
And he does. Of course, Desdemona is innocent of any extra-marital hijinks, but pays a high price for Othello's suspicions, as does Emilia and Othello himself.
(I finished this in December, hence a 2006 book entry.)