Iago is a monster of villainy. He hates Othello for not making him his lieutenant; therefore, he is determined to ruin Othello's life. He plants seeds of jealousy. Iago creates an affair between Desdemona and Cassio. Although there is nothing going on between Cassio and Desdemona, Iago makes it seem that there is an affair between the two of them.
First, he plants the idea for Cassio to ask for Desdemona's help in reinstating Cassio to his position as lieutenant. When Desdemona pleads with Othello for Cassio's reinstatement, Othello becomes suspicious. Of course, Iago is there to plant seeds of suspicion.
Next, Iago takes Desdemona's handkerchief and plants it on Cassio. Iago tells Othello that he has seen Cassio wiping his beard with the handkerchief that was a gift of Othello's love to Desdemona.
Iago then claims that he will speak to Cassio about his affair with Desdemona. Othello is observing from a distance. When Iago and Cassio begin to talk of Cassio's affair with his mistress, Bianca, Othello thinks they are talking about Desdemona. Fury overtakes Othello. In Act 4, Scene 1, Iago states that his medicine is working:
My medicine, work! Thus credulous fools are caught:
And many worthy and chaste dames even thus,
All guiltless, meet reproach. What, ho! My lord!
My lord, I say! Othello!
Iago has planted enough seeds of distrust until Othello turns on Desdemona and smothers her. Iago is a villian. He has done all the damage he can do. Othello has lost his senses to jealousy because of Iago's villainous lies.