Question: why does iago hate cassio?
Answer: Iago claims to resent Cassio because Othello chose Cassio rather than Iago as his lieutenant, in spite of the fact that Cassio has no practical knowledge of battle. Iago uses Cassio in his scheme to destroy Othello; Iago insinuates throughout that Cassio is having an affair with Othello's wife, Desdemona.