I think jealousy and envy are the key motivation and also the key weapon of Iago in bringing about the downfall of Othello. He says that his hatred of Othello is based on the way that Cassio was promoted and not he. At the same time he recognises the jealousy of Othello regarding his wife, and exploits his imagined fears ruthlessly and brilliantly to bring about the play's conclusion.
We also see the jealousy of Bianca when she is asked by Cassio to 'take out the work' - ie. copy the design of the handkerchief he has found in his chamber. She is fiercely in love with him and fears that he is unfaithful. He is dismissive of her attentions and she is jealous of whomever she believes him to be with.
Emilia also shows jealousy towards the way that men's indiscretions are forgiven but women's are not.
Iago is extremely envious of Cassio's new position as lieutenant. Iago assumed all along that he would receive the promotion, and he cannot understand why Othello gives it to someone else. Iago looks on Cassio and tries to determine his strong points, and only ends up making himself more jealous in the process.
Othello kills Desdemona because he is jealous of the affair he is led to believe she is having. Of course, we all know she was innocent, but he never asked her. He just blindly believed Iago's story. Had Othello not allowed jealousy and envy to blind him, he would not have committed murder, and he would have had a happier life.
Iago's jealousy of Othello's imagined affair with Iago's wife, along with Iago's envy of being excluded from the highest levels of military and political influence, serve as a primary motivating factor in Iago's actions. Interestingly, Othello is also envious of those lighter-skinned citizens who have access to higher levels of influence than he. Othello also becomes consumed by jealousy over an imagined affair by his wife, Desdemona. Iago is of course responsible for inciting the marital discord by suggesting to Othello that Desdemona is unfaithful, leaving the rest to Othello's imagination.
Enotes's study guide on Othello provides an excellent character analysis, including discussions on jealousy and envy as motivating factors: http://www.enotes.com/othello/character-analysis
In Othello jealousy is one of the main focused point. Because of the below reasons
1.Iago the villian works out his vilanous intrigues on Othello in order to seek revenge from him.And even for Iago the main reason to take revenge is because of the Jealousy-the green eyed that grows within him day by day of Othello
2.Althogh it is the villanous plans of Iago that led the to tragedy of Othello,it is really the jealousy which made othello to kill Desdemona and also he himself at last because the villanous plans by the villanous Iago was only there to pester the suspicion and there by arousing the jealousy inside Othello's mind.
So it is evident to prove that "Jealosy" is the seed which contributed to this whole tragedy to happen.