Othello is far from blameless, but Iago is so attuned to his weaknesses that he is able to "play him," as it were, to tragic effect. While Iago is certainly one of Shakespeare's most despicable characters, he is also one of my favorites, or perhaps I should say, the most interesting. I'm not sure if he is a catalyst or a "devil," but I do know that it is a question that Shakespeare wants us to ask. Personally, I think Iago is closer to the cause of Desdemona's murder than a catalyst. If not for his machinations, I do not think Othello would have been driven to murder, whatever his shortcomings. As Emilia herself says to Desdemona, after Othello has accused her of infidelity:
I will be hang'd if some eternal villain,
Some busy and insinuating rogue,
Some cogging, cozening slave, to get some office,
Have not devis'd this slander.
Emilia did not know that it was, in fact, her husband who was the "eternal villain," but her comments, I think, describe Iago's actions fairly well.
I would say he is certainly the catalyst for Othello's destruction. He purposely schemes at the beginning of the play how he will hurt Othello. However, each scheme is carefully designed to ensure that Othello or Roderigo or someone else is the one doing the evil acts. Iago only speaks to suggest it. I think this helps the reader realize what Othello's tragic flaws are, therefore Othello is far from blameless. The difference between Othello and Iago is that Othello shows remorse at the end and Iago does not. The absence of remorse is a sign of evil.
I am not sure I agree with this. Yes, Iago cannot be considered fully responsible for what Othello does. However, he is not completely innocent of blame either. If you get the ball rolling, you are also guilty of the consequences in my book.