In the begining of the play, when Iago has his talk with Rodrego about Cassio getting the position, this is the first time that he displays the jelousy he had for Cassio.
Psychologists often make a distinction between jealousy and envy. If you are jealous of another person, you want what he has. If you are envious of another person, you don't even want him to have what he has. Iago is almost more envious than jealous -- or, rather, he seems to suffer from both conditions.
Most of Iago's actions in the play Othello are driven by jealousy. Iago attacks Othello's life because he is jealous of him. He doesn't think Othello deserves the position he has above him. Othello is respected and admired which only fuels Iago's jealousy. Cassio is promoted above Iago which only inflames Iago's jealous nature. Eventually, Iago's jealousy poisons his entire life and the lives of those around him. Jealousy is really the only motivation for Iago's actions. We know that his actions lead to the tragic endings inflicted on the other characters.
how does Iago use jealousy in a desructive manner against Cassio ?