Roderigo is the only person in the play who knows anything about Iago's true motivation. All this time, Iago has been saying things to Roderigo about how he hates the Moor (Othello), and how he only serves him to "serve his turn upon him" (i.e. waiting for his chance to strike). Plus, Roderigo has been giving Iago gifts, with the intent that he deliver them to Desdemona - something that Iago has not been doing. So with Roderigo out of the picture, not only does he get to keep those treasures, but he has one less witness to his crimes to worry about.
Cassio, meanwhile, is in Iago's mind the person who stands between him and his destiny. Cassio got the promotion that Iago wanted, but it's more than that. Cassio has Othello's love and respect. Everybody seems to trust and respect Iago - he is called "honest Iago" throughout the play - but no one is bestowing honors and promotions upon him. So Iago takes out his frustration at this in the form of hatred for Cassio. With Cassio gone, Iago reasons, all this glory should fall to him.
answered 31 May '11, 10:14