Iago's speech in Act V, scene i, lines 11-21 reveals his feelings about Roderigo and Cassio, while at the same time illustrating Iago's character. He says about Rodrigo, "whether he kills Cassio, or Cassio him, or each do kill the other, Every way makes my gain." Iago does not feel gratitude toward Roderigo for his help in the scheme. He knows that if Roderigo lives, he will "call me to a restitution large of gold and jewels I bobb'd (stole) from him as gifts to Desdemona." Iago also reveals in this speech that he wants Cassio dead because he has a "beauty" in his life (an integrity) that makes Iago look inferior in comparison. And, of course, if Othello confronts Cassio, Iagos's treachery will come out. Iago says "He must die." The audience should feel disgusted with Iago's motivations, and it becomes clear that Iago is willing to let people die to cover his lies.
Iago's original plan is to have Roderigo kill Cassio (because Roderigo is jealous of Cassio's supposed affair with Desdemona, whom Roderigo desires). Iago would then dispose of Roderigo himself, or perhaps Cassio would also mortally wound Roderigo, too, in their fight. This is not what happens, however, as Roderigo fails in his attempt to kill Cassio. Iago then stabs Cassio in the leg. Roderigo has been wounded by Cassio, however. Iago has underestimated the fighting skills of Cassio and now Cassio has said that his attacker is in the immediate area. Iago's entire plot is at risk of being exposed. Because Roderigo botched the killing of Cassio, Iago now has to kill Roderigo to keep him from revealing that Iago is behind the entire plot. Iago goes to "check" on Cassio's wound (that he caused!!):
Iago appears and seems to show concern for Cassio’s wound, and when Roderigo appears, Cassio stabs him. The others believe that Iago acts out of revenge for his friend, but Iago’s true motive in killing Roderigo is that he served his purpose and would certainly tell all if he lived. (Enotes)