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Iago is jealous of Othello, but even more than that, he wants to punish Othello for promoting Cassio to lieutenant. Iago expected the promotion and took his anger out on the man he believed slighted him.
Iago has suspicions that Othello (and Cassio, who Othello promoted) slept with his wife.
This, however, is not confirmed and Iago acts out of anger towards both Othello and Cassio; seemingly believing his worth is more than that of the two he tries to sabotage.
This lack of a stable motive makes the character of Iago especially sinister - often referred to as a villainous individual due to this ‘motiveless malignity’