A good theme statement would be:
The concept of jealousy as explored by William Shakespeare in his play, Othello, is a clear indication of how one's inherent fears and uncertainties can be exploited and manipulated by those who are envious of us and how such individuals can deceive us to turn not only against those whom we love, but lead us into betraying our very nature and turn us into monsters, overwhelmed by an emotion that feeds off itself.
The above thesis statement allows the author to discuss how the protagonist is deceived and manipulated by the Machiavellian Iago, resulting in him losing not only his sanity, but also his self-control. It introduces the reader to the idea of Othello's insecurity and allows the author to explore fully that aspect of his nature. Furthermore, it also creates the opportunity for the essayist to delve into Iago's actions and the motives behind them - prominent would be, 0f course, his own insecurity and the idea that he feels threatened and imposed upon by a foreigner who has the audacity to not give him credit where he feels credit is due.
Other obvious links to this theme statement would be Roderigo's obvious jealousy of both Othello and Cassio. Iago's envy of Cassio - all linked to the characters' deep feelings of uncertainty and insecurity.
To write a good thesis statement about jealousy in Othello, think about how the characters show their jealousy, what forces characters to act out of jealousy, and what characters are not affected by jealousy. A good statement might be:
Through Iago's and Othello's actions and dialogue in Othello, it is clear that jealousy can cause individuals to marginalize others, as illustrated by racial and sexist prejudices depicted in the text.
We could make the case that in the play, Iago is incredibly jealous of Othello. Othello is an outsider yet was able to rise in the military and gain respect from the nobles. Othello also won the most desirable woman in Venice. Iago constantly speaks offensively about Othello and particularly notes his race. We might argue that Iago is self-conscious about his own inadequacies. Perhaps Othello does the same to Desdemona; when he suspects her of having an affair with Cassio, he uses sexists slurs to marginalize her because he is lashing out from his own insecurities.