With this final line, Iago is saying that he will not reveal anything about his plot that his wife Emilia has not already revealed. Othello has just confessed to murdering his wife and conspiring to have Cassio murdered. When Cassio points out that he never did anything to Othello to justify this, Othello agrees, and encourages Cassio and Lodovico to ask the "demi-devil" Iago about his role in the proceedings. With this quote, Iago swears not to reveal his plot. To this, Gratiano replies that "torment shall ope your lips," meaning that Iago will be put to torture to get him to speak, and this is an important point. On the one hand, many have suggested that Iago does not receive justice at the end of the play, whereas Othello, Emilia, and Desdemona die. But this line suggests Iago is destined to live out however few days he has left in agony and shame. But it also allows Iago not to reveal the real reason for his horrible plot, which many commentators have surmised, goes well beyond redressing the small indignity of being passed over for promotion by the Moor.
In Othello, Iago and Roderigo are the first characters the audience is introduced to. As much as Iago reveals his intentions to the audience and Roderigo, when he says in Act I, scene i, line 42, "I follow him [Othello] to serve my turn upon him," he conceals his true nature to the other characters. They do not see through his deceit, and he even appears to be the most "honest" among them. Iago will take full advantage of the fact that, as he says in line 66 of Act I, scene i, "I am not what I am."
Iago goes to great lengths to turn everyone against Othello and, at first, it seems that the only reason he does this is because he, who has no loyalty to anyone, even his own wife, feels betrayed by Othello. Iago thinks he should have been Othello's first choice for lieutenant. What's more, Iago is shocked that Othello chose Michael Cassio who has never been in battle and is a "bookish theoric," (24) whereas Iago has proven himself in Rhodes and Cyprus and on other battlefields.
As the play progresses, it becomes apparent that Iago's resentment builds and he plots far worse deeds, not satisfied until Othello is ruined. He does not stop to think of the other lives that are equally, and tragically, ended as he focuses on that which would cause Othello the most grief. In doing so, Iago becomes paranoid, even believing his own delusions, thinking that Emilia, his wife, has been unfaithful with Othello, when there is no reason or circumstance to make him think that.
Iago fools himself into thinking that the satisfaction he will feel after destroying Othello will be the resolution that he is seeking. By the time he has set up everyone, making them look guilty, he knows that his plan may go awry but is excited at the prospect when he says in Act V, scene i, "This is the night that either makes me or fordoes me quite" (128-129). The fact that he feels no remorse after Emilia and Desdemona die speaks to his ability to remove himself from his actions.
Significantly, Iago's last words from Act V, scene ii, when he says, "Demand me nothing. What you know, you know. From this time forth I never will speak word" (306-307), indicate that, even though he has been taken prisoner and has been revealed as a villain, Iago retains some control, or thinks he does. There is some satisfaction for him in knowing that, although Othello knows that Iago conspired to ruin him, he remains "ensnared" (line 305). Iago feels that he has achieved what he set out to do.