In Othello, Iago will go to any lengths to "serve my turn" (I.i.42) on Othello who has passed him over for lieutenant and he has no concerns in using any other characters to strengthen his case against Othello personally. Iago is manipulative and takes Roderigo into his confidence from the beginning so that Roderigo is fooled into thinking that Iago respects him and understands how much he loves Desdemona. By exposing his need to exact revenge on Othello, he incites Roderigo to do the same as Roderigo has long since loved Desdemona but has been repeatedly rejected. Even Brabantio, Desdemona's father, is not inclined to listen to Roderigo's nonsense as he has warned him previously "not to haunt about my doors" (97). Therefore, Roderigo's desperation is apparent and Iago will use it to his full advantage, encouraging him and making false promises to him.
When Iago's first plan fails and Othello and Desdemona declare their mutual love for each other, Roderigo is devastated and wants to end his life because "death is our physician" (I.iii.320). Iago feigns interest in Roderigo's grief and offers his advice, telling Roderigo that they can both have their revenge on Othello and that they can make their plans together. This encourages Roderigo who misinterprets Iago's self-serving interests for friendship. As soon as Roderigo leaves, Iago says, "Thus do I ever make my fool my purse" (377) revealing that he is able to profit from Roderigo's unfortunate situation. He feels no compassion for Roderigo but only sees benefit to himself.
In Act II, Iago will continue his manipulation of Roderigo by convincing him that Desdemona is actually in love with Cassio and that Roderigo needs to remove "the impediment" (II.i.273). Roderigo agrees to do it because Iago convinces him that this will further his cause. After he leaves, Iago furthers his own plot against Cassio and his intention to get him drunk and the audience is made more aware of his feeling for Roderigo when he expresses the hope that "this poor trash of Venice" (297) agrees to help him trick Cassio. Still later, in Act II, scene iii he says, "my sick fool Roderigo" (47). Ultimately, even though Roderigo will admit that he has no compunction to kill Cassio, Iago will convince him that he should.
In the first act of Othello, Iago makes it very clear when he is speaking to Roderigo that he is out for himself and that he plans to bring down Othello by any means possible. In order to enlist Roderigo's help he plays on his vanity and his weakness: his desire for Desdemona. Both she and her father haver rejected Roderigo as a suitor, and his pride is hurt. Iago promises Roderigo that if he helps in Iago's plot, that Roderigo can have Desdemona for himself. Roderigo is foolish enough to believe him, but Iago has no intention of doing anything for anyone but himself, something Roderigo should have realized from the start. Though it is clear to the audience that Iago despises Roderigo, he really ony admits it out loud in Act 2, Scene 1 when he calls Roderigo "this poor trash of Venice" in line 325, clearly letting the audience know that he has no love for Roderigo and only tolerates him because he thinks he can use Roderigo to his own advantage.