Iago is one of Shakespeare's most evil villians. His betrayal of his commander, his friend and his own wife spring from his jealousy of being overlooked for a promotion. He immediately begins spreading the idea that the Moor Othello has used witchcraft in convincing Desdemona to marry him. When this does not work in having Othello demoted, he uses his friend Roderigo to romantically pursue Desdemona, telling him that she does love him. Iago must keep the wealthy Roderigo interested so that he has a continual flow of money.
As this plan becomes less likely to succeed, Iago turns to using Cassio, a friend of Othello's, against him by insinuating that Cassio and Desdemona are having an affair. He even enlists his own wife, Emilia, to steal Desdemona's scarf for Iago's evil purposes. When Cassio turns up with the scarf, Othello is convinced. Iago has convinced him that Desdemona is cheating; Othello kills her. However, Iago is not finished. When Emilia threatens to blow his cover, Iago kills her as well.
The tragic outcome has several dead, including Othello by his own hand, and Iago a caged animal. The betrayal ended with himself.
Betrayal is a main theme in Othello and is revealed through the actions and often irrational behavior of the characters. First of all, there is the perceived betrayal of Desdemona when she marries Othello without her father's knowledge or consent. Ironically, Brabantio warns Othello, when he says, "She has deceiv'd her father, and may thee," (I.iii.293). He is suggesting that she has therefore shown a potential for disloyalty. Immediately after this warning, Othello significantly leaves Desdemona in the care of "Honest Iago," (294); the same Iago who alerted Desdemona's father to her marriage in the first place so that he could cause a disturbance and who even admits that he merely shows solidarity with Othello in order to "serve my turn upon him," (I.i.42); in other words, he acts for his own benefit.
Iago feels slighted, offended and even betrayed, by Othello's choice of Michael Cassio as his lieutenant, when he believes he is far more qualified and deserving and intends to make , not only Othello, pay for his own (Othello's) apparent, poor choices. Therefore, Iago ruins Cassio's reputation for his part in being chosen by Othello. Iago is also delusional believing that Othello has been unfaithful with Iago's wife and this is just another reason why Iago ensures everyone's downfall. He is determined to exact his revenge on anyone who has disappointed him or benefited when Iago feels he should have been the one to benefit. There is no foundation on which he bases his claim that Othello has been unfaithful with Iago's wife but this does not stop him making the claim.
From the perspective of destroying Othello, it will not be enough to cause Othello pain. He intends to incite Othello sufficiently until Othello does something so heinous and unforgivable which is what he means when he refers to, "a jealousy so strong That judgment cannot cure." (II.i.295-296). The judgment to which Iago refers is rational judgment and discernment.
It is Iago's intention to use whatever opportunity he can to make Othello doubt his friends and trust Iago without hesitation. This will enable him to completely discredit Othello. For example, even though Othello must know in his heart that Desdemona would not betray him, he is so affected by Iago's efforts and his ability to manipulate him, that all rational thought abandons Othello. At the end, even Othello can hardly believe what he has done because he thinks of himself as, "One not easily jealous, but, being wrought, perplexed in the extreme..." (V.ii.348-349) meaning that he has acted out of character and only in the interests of honor. Iago has successfully destroyed everyone, himself included.