In many respects, this is the central tenet of the drama. Why do people commit evil? Certainly, I don't think that there is any doubt that Iago is malevolent. The question that the reader is left to ponder is what motivates Iago to become so intent on destruction and evil. I think that a surface answer would be that Iago is disappointed that Cassio is promoted over him. This envy and sense of resentment is the initial driving force behind what he does. On another level, Iago represents the bitterness of the disenfranchised. He has been denied and deferred and does not subscribe to the idea that "waiting for his turn" is the correct path. He is filled with an intensity of anger towards anyone who he sees as happy and content. They become his enemy. Even his own wife, Emilia, is killed because she is more centered than he is, for while she is a part of his plan, she recognizes the direction of her husband's machinations and no longer wants a part in it. Over the course of the drama, Iago becomes motivated and driven by the idea that anyone who has found happiness while he has come across pain and suffering must suffer, experiencing for just a moment the torment and bitterness that he lives with each day. In this, Iago's motivation to deliver evil and malevolence to others is an extension of his own subjectivity, and a bizarre way of appropriating the world in accordance to it.
Iago's motivation in Shakespeare's Othello is akin to that of Ravana's in the Indian epic - Ramayana. Ravana wanted to win the heart of Seeta and marry her, but Rama's better capabilities woe her instead. That burns Ravana's heart in jealousy and feels like his pride and manhood crushed by Rama' marrying Seeta. Iago, besides other things, cannot digest the fact that Desdemona is the wife of Othello. No doubt, he also dislikes Othello being made the General. That way, the glory and the girl have both been snatched away by Othello. Obviously, the resentment and the mental suffering dominates Iago's psyche. He is desirous of both - Generalship and Desdemona, the glory and the girl. But only thing he could do now to achieve the goal is first to seperate Desdemona from her husband by hook and crook and later remove Othello. The three tempting words - wealth, wine and woman - raise the monster out of a law abiding citizen and turn him into a criminal. Similar is the case with Iago. In his case, 'wealth' is the post of General, 'wine' is his courage and the 'woman' is Desdemona, Othello's wife. That could be the motivating factors in Iago's villainy.
In the play Iago reveals three things that are his motivations:
1) He has heard rumours and believes that Othello and Cassio have slept with his wife.
2) He is angry at Othello for not promoting him, and at Cassio for taking the promotion that he wanted.
3) He says he lusted after Desdemona. Though he also says that he wants to get back at Othello for supposedly sleeping with his wife and making Desdemona betray Othello sexually is the best way for him to do it.