Why did Iago want to anger cassio?
Iago's goal was not to anger Cassio. In "Othello", Cassio had gotten the position of lieutenant that Iago had felt that he had been more deserving of. This is why Iago wanted to ruin Othello and Cassio and so it was less about trying to make Cassio angry than it was that he wanted to dispose of him. This is how his scheming began.