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Iago hates Othello and resents Cassio because he is job competition. Thus, he plans to take both men out at the same time. He plans to suggest to Othello that Cassio (quite the ladies' man) is having an affair with Othello's wife (Desdemona). Breaking up the marriage would assist in his revenge against Othello, and Cassio's involvement would rid him of job competition. Iago figures it's a win/win proposition.