One is a puppet master; the other, a puppet. Iago plays Roderigo like a puppet. He uses him and his money for his own ends, chief of which is to harm Othello. Othello has supposedly humiliated Iago by appointing an inexperienced foreigner, Cassio, as his lieutenant and ignoring him in spite of the fact that he has been his loyal, brave, and experienced servant, and Iago seeks vengeance for this.
From the outset, Iago has no qualms about using the foolish and gullible Roderigo, who virtually becomes the deceitful and sly manipulator's slave. He does Iago's bidding at a whim, without much question as he did when, in Scene 1 of Act I, he was instructed to deceive Brabantio into believing that Othello had abducted his daughter and was in the process of abusing her.
Furthermore, Iago is in control of Roderigo's seemingly ample purse and consistently asks him for money and Roderigo, like a dunce, willingly complies to his requests. The sole reason for all his acquiescence is that Iago is dangling the fact that he will help the besotted dolt win Desdemona's affection like a carrot in front of him. Roderigo is obsessed with the beautiful Desdemona, Othello's wife, and would do everything to have her.
Iago remorselessly uses this promise to exploit Roderigo and he declares, at various stages in the play, what he thinks of Roderigo and why he is playing him for a fool. For example, at the end of Act I, Scene 3:
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.
In these lines, Iago most pertinently states that he would not have spent time with such a foolish person if it were not for his own "sport and profit." It is clear that he does not see Roderigo as a friend but rather as a tool that he can use for his own pleasure and benefit. The fact that he refers to Roderigo as a "fool" whom he makes his "purse" further emphasizes the fact that he is shamelessly using him.
Roderigo, however, seems to believe Iago actually has the power to ensure him success in getting Desdemona's attention and winning her over. He believes Iago is his friend, consistently follows his advice, and is even prepared to disguise himself and undertake an arduous sea journey to a dangerous place just to be close to Desdemona and improve his lot. One can only pity such a gullible sacrifice. When Roderigo eventually realizes that he has been mislead by empty promises, most of the damage has already been done and he has lost practically his entire wealth to Iago.
In this regard, then, Roderigo's confrontation with Iago is a sad failure because he is once again persuaded by the glib and masterful schemer that all is going well and that whatever he, Roderigo, may believe, is a figment of his imagination. So persuasive is Iago that Roderigo, once again, does his bidding and goes out of his way in trying to kill Cassio as instructed.
In the end, Roderigo pays a terrible price for his subservience and his ignorance. Not only does he lose his material wealth and his chance at love, but he also loses his life at the hands of the one he trusted the most, Iago. It is ironic that his intention to expose his malicious master only becomes evident after his own death.