Emilia makes the following remark in Act 3, scene 4:
But jealous souls will not be answer'd so;
They are not ever jealous for the cause,
But jealous for they are jealous: 'tis a monster
Begot upon itself, born on itself.
She is saying this in response to Desdemona's statement that she has never given her husband cause to be jealous. Emilia is saying that those who are by nature jealous do not need a reason for their ill feelings of distrust. They are the way they are because that is their character. This means that Othello does not need any reason to be suspicious of Desdemona, because he is a naturally jealous person.
Emilia says that jealousy is a beast that creates itself and feeds on the negative emotions and cynicism it evokes. The implication is that the more one doubts, the more insecure one becomes. In Othello's situation, he becomes more and more suspicious of Desdemona's actions, especially now that she has lost his precious handkerchief and is insisting that he speak to Cassio about his possible reinstatement.
Emilia's remark comments on the major themes of jealousy, envy and revenge. She is, ironically, also referring unknowingly to the sentiments her husband feels towards not only Othello but also Cassio and Desdemona. It is clear that Iago's jealousy feeds on itself. He is envious of Othello who, in spite of being a foreigner, has attained the highest position in the Venetian army. It is clear that Iago despises the fact that Othello, a Moor, has achieved a position that he obviously believes he would be more suited for.
To add insult to injury, Othello is also his superior and has denied him the promotion he has so desperately been seeking. The general has, instead, offered the position of lieutenant to Cassio, who is an inexperienced Florentine—another outsider. Iago's resentment is palpable, and he wants to punish both his leader and the innocent Cassio for what he believes is a humiliating dishonor.
Iago resents Desdemona's innate goodness. Because he is naturally evil, it is beyond his understanding that someone can be as virtuous as she. His despicable nature makes him turn against her. Furthermore, he finds Desdemona desirable, and is jealous of the fact that Othello has actually successfully wooed her.
In addition, Iago suspects that Othello has also had inappropriate relations with his wife, Emilia. He has no proof but, as Emilia states, he does not need any, since he is driven by his own inner malice. Iago's malevolence becomes the source of all the drama in the play. When he discovers that he cannot compromise the general's position, he turns to others to do his dirty work for him.
Iago plays on Roderigo's own feelings of envy against the general and persuades him that he can help him successfully woo the beautiful Desdemona and win her love. Roderigo becomes Iago's gullible puppet and is exploited and used by the master manipulator to do his bidding. He realizes, too late, that he has been used.
In the end, it is Iago's malice that leads our protagonists to their doom. He, in the end, faces the wrath of Lodovico, the slain Desdemona's cousin, who tells Iago in the final lines of the play:
O Spartan dog,
More fell than anguish, hunger, or the sea!
Look on the tragic loading of this bed;
This is thy work: the object poisons sight;
Let it be hid.
He then instructs the governor of Cyprus to punish Iago for the evil he has perpetrated:
To you, lord governor,
Remains the censure of this hellish villain;
The time, the place, the torture: O, enforce it!