Iago is Othello's ensign and the villain of the play: the subordinate who, resentful at having been passed over, leads the general to ruin and causes the deaths of Desdemona, Emilia and Roderigo with insinuations and lies, while Othello and others call him honest.
In the very first scene he tells Roderigo the motive: three nobles of the city went to ask Othello to name Iago lieutenant, and Othello chose Cassio. He says he serves the general only for his own interest ("In following him, I follow but myself") and warns: "I am not what I am."
His method is to use others. Roderigo gives him money and jewels in hope of winning Desdemona, and Iago remarks: "Did you see how a dupe is used to stuff a dupe's purse?" He makes Cassio drink and lose his post and, in Act III, Scene 3, plants doubt in Othello by repeating his words, until he goes on to outright lies, with a dream and a handkerchief. Of the handkerchief, which Emilia hands him, he says: "I will drop the handkerchief in Cassio's chamber."
In Act V, he sends Roderigo to kill Cassio, wounds Cassio from behind and stabs Roderigo; in an aside, he had admitted that Roderigo alive would demand from him the gold and jewels. He kills his own wife when she reveals the handkerchief, and flees. Captured, he says: "Demand nothing of me. What you know, you know." Lodovico orders that he be kept alive for torment.