Iago wants to revenge because of his jealousy that Othello has been chosen by lieutenant over him. He believes that it is what he should be in charge, not Othello, the moor. This creates anger to Iago. Because Iago who needs to revenge, so he started an evil scheming to destroy Othello. Moreover, he use every opportunity to success his plan. Until he found out Othello's weakness of jealousy, he think of many ways and leads Othello to trust him and then use his own hands to murder his wife. He deceives him that his wife and Cassio are having an affair so as to destroy his wife and even the lieutenant. By the way, Iago resents Cassio who as I said he chose Othello as a lieutenant, therefore, Iago pretend to befriend with Cassio and use Roderigo to access his financial benefit as well as to removed Cassio by making them quarrel.
It is beyond our imagination that such a cunning person would have lots of trick to satisfy his dirty work. It is clear that Iago is just like an evil and ignoring people's life and feelings. It shown that he has no conscience to destruct not only one's love and position, but also one's live, causing the downfall from Othello and render a tragedy at the end of the scene.