Iago has no conscience. He is an angry man and is happy to take down everyone around him to get what he wants: revenge. It is in Act 1, Scene 3, that he devises his evil plan. Here we can see inside Iago’s mind. It is easy to see that his primary motivation is jealousy: jealousy that Othello may have slept with his wife, and jealousy that Othello chose Cassio over him. As he plots his revenge, it is clear Iago respects and cares for no one. (Act 1, Scene 3, 378-381)
I hate the Moor, And it is thought abroad that twixt my sheets H’as done my office. I know not if’t be true, But I, for mere suspicion in that kind, Will do, as if for surety. Iago states here he suspects Othello may have slept with his wife. He is not sure of this, but declares that surety is not necessary. I believe Iago is not so much concerned with his wife being unfaithful, but that he can’t stand the thought that it may have been with Othello. (Act 1, Scene 3, 381-382)
He holds me well; The better shall my purpose work on him. This shows how conniving Iago is. He will use the fact that Othello trusts him to get his revenge. (Act 1, Scene 3, 383-385) Cassio’s a proper man. Let me see now; To get his place, and to plume up my will In double knavery. How? How? Let’s see. Here “double knavery” means to pull off one stunt and obtain two desired outcomes – to get Cassio’s position (which he felt he deserved) and to make himself appear respectful for his ego’s sake.