In Shakespeare's, Othello, perhaps Iago is the worst of the Bard's villains. Iago is jealous because he was passed over for a promotion; when Cassio is promoted instead, Iago plans to destroy the man. And to do so, he strikes where Othello is most vulneralbe—his heart. For not only is Othello newly married and in love, but he is ferociously jealous and, with help, loses his faith in Desdemona.
To destroy Desdemona and Cassio (who are really only casual acquaintances), Iago plants false evidence and spreads untruths so it seems as if Desdemona and Cassio have been sleeping together.
To get Roderigo to attack Cassio, Iago lies and says that Desdemona is tired of Othello and is having an affair with Cassio—their lips are so close, Iago says, they might as well be kissing.
Lechery, by this hand; an index and obscure prologue
to the history of lust and foul thoughts. They met so near
with their lips that their breaths embraced together.
Villainous thoughts, Roderigo! (II.i.268-271)
Roderigo is easily antagonized. If the men fight, Iago believes that Cassio will lose his position in the military. Then, Iago gets Cassio drunk, who gets into a fight with Roderigo. As punishment, he is removed from military service.
Cassio, I love thee;
But never more be officer of mine. (II.iii.241-242)
Distraught, Cassio looks to find a way back into Othello's good graces; Iago encourages him to speak with Desdemona. He will use this time that they spend together to infer that they have been meeting behind Othello's back. At Cassio's request, Desdemona replies:
Be thou assured, good Cassio, I will do
All my abilities in thy behalf. (III.iii.1-2)
Iago tells Cassio to leave quickly. The act that he seems to flee for guilt's sake, and in light of Desdemona's plea on Cassio's behalf, Iago will make the Moor think Cassio and his wife are having an affair. (Meanwhile, Iago finds Desdemona's lost handkerchief and plants it in Cassio's room. Cassio gives it to Bianca who he fancies—not knowing whose it is. Iago will use Cassio's possession of the article to prove Desdemona's unfaithfulness, when Othello asks about its absence.)
By Act Four, scene one, Iago accuses Desdemona and Cassio of adultery. Iago does nothing to calm the Moor. His plans are falling together perfectly. Othello is enraged—how could their actions be innocent, he asks?
Naked in bed, Iago, and not mean harm!
It is hypocrisy against the devil:
They that mean virtuously and yet do so,
The devil their virtue tempts and they tempt heaven. (8-11)
Soon, Iago asks Cassio about Bianca, but Othello (eavesdropping) believes the talk of sex is about his wife, and he vows revenge.
Dost thou hear, Iago?
I will be found most cunning in my patience;
But—dost thou hear?—most bloody. (102-104)
Othello is called away, and Iago convinces Roderigo that if he kills Cassio, he will then have Desdemona. (Cassio ends up killing Roderigo instead.)
At the same time, Emilia tries to convince Othello of his wife's fidelity. He cannot be persuaded. Othello strangles Desdemona, who swears her innocence and love with her last breath. Emilia exposes Iago as a villain. Iago kills his wife. Othello, devastated, kills himself:
Then must you speak
Of one that loved not wisely but too well;
Of one not easily jealous, but, being wrought,
Perplex'd in the extreme; of one whose hand,
Like the base Indian, threw a pearl away
Richer than all his tribe... (V.ii.392-397)