Why does Iago suggest Roderigo kill Cassio?

Iago plans to get revenge on Othello for possibly sleeping with his wife and promoting Cassio to lieutenant. In order to do so Iago manipulates Roderigo, who is infatuated with Desdemona, to help him carry out his plan. Iago continually persuades Roderigo into giving him money under the guise that Iago is helping him win Desdemona's heart. Iago successfully convinces Roderigo that Michael Cassio has wooed Desdemona so that Roderigo views him as a threat. Iago then suggests that Roderigo kill Cassio. Iago believes that Cassio will be able to kill Roderigo in a fight, which will relieve Iago from paying back the money he owes Roderigo. If Roderigo is able to kill Cassio, Iago's treachery will not be exposed, and he will have his revenge on Cassio. In act 5, scene 1, Iago explains what he has to benefit from Roderigo and Cassio fighting. In an aside, Iago says,

Now, whether he kill Cassio Or Cassio him, or each do kill the other, Every way makes my gain. Live Roderigo, He calls me to a restitution large Of gold and jewels that I bobbed from him As gifts to Desdemona. It must not be. If Cassio do remain He hath a daily beauty in his life That makes me ugly. And besides, the Moor May unfold me to him—there stand I in much peril. No, he must die (Shakespeare, 5.1.13-23).