This scene is framed by Iago's comments on the importance of this night. Before the action starts, he tells Roderigo: "It makes us or it mars us, think of that, / And fix most firm thy resolution" (4–50). To Roderigo, Iago is saying "Be brave, kill Cassio, and you will have Desdemona." To himself, he is saying "Be brave, make sure Roderigo, Cassio, and Desdemona die, and you will have your revenge on Othello."
Roderigo is still wavering, nursing his last flicker of moral sense: "Be near at hand, I may miscarry in't" (6). The comfort of the coward is in belief that someone will protect him, but by agreeing to rely on Iago to make the decisions, Roderigo abdicates responsibility for his own actions and is led out to kill a man he doesn't hate for a cause he no longer thinks can be won.
Iago wastes no emotion on the prospect of Roderigo's death but acknowledges a certain satisfaction when he thinks of Cassio dead. There is the old fury of jealousy against Cassio who has the good opinions of every one, including Othello (until Iago's duplicity, that is). The unfairness of Cassio's happy life rankles Iago as evidenced in his first speech in Act I and continues to frustrate him now: "[I]f Cassio do remain, he has a daily beauty in his life, that makes me ugly" (18–20). Add this to the need to prevent Cassio talking with Othello, and his death will be Iago's pleasure.
