Shakespeare uses various instances throughout the play to illustrate Iago's malice and how easy it is for him to maneuver Roderigo into doing his bidding. We learn from the outset that Iago has some control over Roderigo, especially his money, for Roderigo tells him in Act I, scene 1:
...I take it much unkindlyThat thou, Iago, who hast had my purseAs if the strings were thine, shouldst know of this.
Through their conversation, we discover that Iago is deeply embittered about the fact that his general, Othello, has appointed an outsider, Cassio, as his lieutenant instead of extending the honor to him, who has been his loyal and trusted servant. He has embraced Roderigo as a friend, more like a puppet, to assist him in avenging his humiliation for Othello's snub. The two are planning to sully the general's name and the idea is to awaken Brabantio, a senator, and inform him that Othello has abducted his daughter, Desdemona, and is at that very moment abusing her.
Iago hopes that this will result in Othello's dismissal and possible imprisonment, which will be an immensely gratifying situation to him. At the same time, he has convinced Roderigo that he will also profit from this malicious venture. Brabantio's address to Roderigo makes it clear that he had been attempting to woo the beautiful Desdemona but has been banished by Brabantio from his house and denied any contact with her. Roderigo is obviously infatuated with her.
We later learn that Iago has been using Roderigo's desire for Desdemona to manipulate the foolish young man, who seems to have more money than common sense. When their plot to have Othello shamed fails hopelessly because Desdemona has come to his defense and expressed her love for him, Roderigo is wholly distraught since Othello was to leave Venice and travel to Cyprus with her. Not only has he lost the chance of wooing her, but she will also be gone—a reality that the lovesick fool cannot bear. He tells Iago that he will commit suicide.
Iago dismisses Roderigo's threat that he will drown himself as a preposterous notion. He urges him to fill his purse with money (at least eight times) so that he may win Desdemona's hand. He tells Roderigo, in part, the following:
...Itcannot be that Desdemona should long continue herlove to the Moor,
...nor hehis to her: it was a violent commencement, and thoushalt see an answerable sequestration:
These Moors are changeable intheir wills: ... —the foodthat to him now is as luscious as locusts, shall beto him shortly as bitter as coloquintida. She mustchange for youth: when she is sated with his body,she will find the error of her choice: she musthave change, she must:
if sanctimony and a frail vow betwixtan erring barbarian and a supersubtle Venetian nottoo hard for my wits and all the tribe of hell, thoushalt enjoy her;
Roderigo seeks Iago's assurance that he will hold good his promise to, as it were, deliver Desdemona to him. Iago says:
Thou art sure of me:—go, make money:—I have toldthee often, and I re-tell thee again and again, Ihate the Moor: my cause is hearted; thine hath noless reason. Let us be conjunctive in our revengeagainst him: if thou canst cuckold him, thou dostthyself a pleasure, me a sport. There are manyevents in the womb of time which will be delivered...
This is the foundation on which Iago's manipulation of Roderigo rests. He promises the desperately infatuated wreck that he will ensure success in his desire to win Desdemona's hand. All that Roderigo has to do is to provide him with money. He should also travel to Cyprus in disguise, where the two of them will plot Othello's cuckolding. Roderigo will have Desdemona and Iago his revenge. Roderigo then sets off to sell all his land and Iago states:
Thus do I ever make my fool my purse:For I mine own gain'd knowledge should profane,If I would time expend with such a snipe.But for my sport and profit.
Roderigo is a mere plaything in his hands, there for his pleasure, and he intends to get rid of him later.
Iago's manipulation of Roderigo continues later in the play. He informs Roderigo that Cassio and Desdemona are having an illicit affair and that he should be removed. When Roderigo expresses doubt about his assertions regarding Desdemona's virtue, Iago tells him that she is driven by lust and that Cassio is a lecher. He asks him to draw out and anger Cassio, so that he may be dismissed.
...Soshall you have a shorter journey to your desires bythe means I shall then have to prefer them; and theimpediment most profitably removed, without thewhich there were no expectation of our prosperity.
The two men succeed in this enterprise and Cassio loses his post. Roderigo, however, loses patience and has no money and wants to return to Venice. Iago tells him to be patient. He later confronts Iago, stating that he feels that he is playing him for a fool. He threatens to approach Desdemona and expose Iago's malice. Iago, however, charms the blustering clod and informs him of a new plan: Othello would be leaving for Mauritania and Cassius would be appointed in his post in Cyprus. Desdemona will leave with Othello, removing her even further from Roderigo. To prevent this from happening, they need to get rid of Cassio. The gullible Roderigo once again takes the bait and promises to assist Iago.
When Roderigo confronts Iago and threatens to expose his scheme, he essentially signs his own death warrant, for, later, after the altercation with Cassio in which the erstwhile lieutenant and Roderigo are both injured, Iago surreptitiously appears and kills Roderigo, thus removing the threat. Later, though, a letter implicating Iago is found on Roderigo's person, which, with Emilia's accusation, leads to his arrest, incarceration and torture.