Lago convinces Roderigo that Cassio has been promoted to be the new governor of Cyprus, which means that Othello and Desdemona will soon be leaving the island. He tells Roderigo that, if he kills Cassio, Othello and Desdemona will have to stay in Cyprus longer, giving Roderigo time to make his moves on Desdemona.
You REALLY needed to provide more info so that's the best answer I'll give you lol
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