I constantly tell my students that Iago is one of the most villainous characters in all of literature. In fact, they spend weeks researching and examining his motives and ultimately conclude that he has no good reason for carrying out the plan that he devises. We read criticism from different Shakespeare scholars, and their ideas regarding Iago's motives differ greatly. Inevitably, my students are usually intrigued by the fact that people (the critics, in particular) are still so fascinated by Iago's character.
Iago's ability to manipulate Othello, to create turmoil and chaos in his life with such unsuspecting grace make him a most memorable villain.
Iago is an insidious villain because he has a way of getting inside the head of others. He preys on Othello's insecurity over his wife. Its so easy for him to create a cloud of suspicion about Desdemona's fidelity that we forget how masterful Iago truly is in this play.
Iago controls his emotional responses, he does react in anger, but plots his revenge. What he does to Othello, and why, is no secret to the reader, he has no idea that he is Iago's target of vengeance.
"He is disgruntled at having been passed over for promotion, and he sees a chance to get back at both Othello, who has slighted him, and Cassio, the mocking symbol of that slight. Second, he suspects that Othello has engaged in adultery with his wife, Emilia. He mentions this on two occasions: "I hate the Moor, / And it is thought abroad that 'twixt my sheets / He has done my office" (I.iii.388-390)"
Iago's ability to unravel Othello is done with such skill that, as a villain, he must be admired.
"Regardless of the degree to which Iago is to blame for Othello's downfall, he remains one of Shakespeare's most villainous creations, variously described as a brilliant opportunist taking advantage of the chances presented to him, as a personification of evil, and as a stock "devil" or "vice" figure."