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.