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Iago wants to get revenge on Othello for giving Cassio the position of lieutenant instead of him, which has a lot to do with why Iago hates Othello so much. One thing in particular he does, in order to carry our his revenge, is to make Othello believe that Cassio is sleeping with Desdemona. This is definitely not the truth, but Iago is so convincing that Othello has no choice but to believe him. There is a dialogue between Iago and Othello that shows how manipulative Iago is when speaking about Desdemona: .
Iago: "Did Michael Cassio, when (you) woo'd my lady, know of your love?- .
Othello: "He did from first to last. Why dost thou ask?-.
Iago: "But for the satisfaction of my thought, No further harm.
Othello: "Why of thy thought Iago? (III, iii, 94-98) .
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This dialogue shows the control that Iago has over Othello. Iago purposely makes it seem like he asks that question without meaning much by it in order to provoke Othello into questioning him further. Furthermore, Iago also plants Desdemona's handkerchief in Cassio's cabin knowing that Othello will eventually find it and think it was Cassio who took it: "Thou saidst (O, it comes o'er my memory, As doth the raven o'er the infectious house, Boding to all) he had my handkerchief)- (IV, i, 19-22). Iago uses persuasive words to make Othello believe these illusions, while making Othello trust in him because only a good friend would break this kind of news to another friend. If Othello trusts him, then Iago will have an easier time deceiving him. .
Othello's personality transforms with the false suggestions placed upon him by Iago, and as a result he becomes more angry and vulnerable and ultimately goes insane. Othello, towards the end of the play, does not have the patience he used to. He hurries through many decisions that need to be made, such as the one to kill his wife: "Get me some poison Iago, this night. I'll not expostulate, with her, this night lest her body and beauty unprovide my mind again, this night Iago- (IV, i, 204-206).
