Why, in the play Othello, is Roderigo angry with Iago?

Roderigo is angry because he believes that Iago has been using him without any reward to himself. He has been giving Iago all his money in the hope that he would enable Roderigo to win Desdemona's affection since he is infatuated with her. At this point, there has been no improvement in his situation and he is frustrated at Iago's apparent delay. He tells him in Act 4, scene 2:

I do not find that thou dealest justly with me.

When Iago asks him why he makes this accusation, he says:

Every day thou daffest me with some device, Iago;
and rather, as it seems to me now, keepest from me
all conveniency than suppliest me with the least
advantage of hope. I will indeed no longer endure
it, nor am I yet persuaded to put up in peace what
already I have foolishly suffered.

Roderigo is clearly upset and tells Iago that he provides him with some kind of story or scheme but that it appears to him as if he is, in fact, keeping him from achieving his goal, which is to be with Desdemona. He tells Iago that he will no longer stand for it and he is not convinced enough about Iago's intentions. He is also not prepared to easily let go of what he has already foolishly given up and endured.

He, furthermore, tells Iago that he has already heard too much from him since his words and his actions don't match - i.e. he does not put his words into action. He also accuses Iago of having spent all his money and jewels and that he has bankrupted himself, yet, he has received no reward. Iago had received enough money and jewels from him to corrupt a politician and had promised that he had given the same to Desdemona. In return, Roderigo would receive attention and respect from Desdemona, but he had received nothing.

When Iago expresses his disapproval of Roderigo's accusations and tells him to be gone, he tells him that he cannot and will not go. He threatens Iago that he will confess what he has done to Desdemona and reclaim all his jewels. If he is not satisfied in this, he will seek restitution from Iago. 

Iago cleverly manipulates Roderigo by sweet talking him and saying that he recognizes true courage, purpose, and valour in him. He tells Roderigo that if he commits to one more action, he will truly have Desdemona for himself the next night or he could take his life, if not.

The gullible Roderigo is appeased and agrees to prevent Cassio to take Othello's place when he supposedly leaves for Mauritania. If Cassio is not fit to replace Othello, the general will have to stay in Cyprus and so too will Desdemona. And so, once again, Roderigo is manipulated and becomes Iago's convenient puppet.

In the end, though, it is Roderigo's damning letters, found on his person after he has been killed by Iago, that condemns the master schemer and exposes him for a fraud, a malicious liar, and a remorseless cheat.