Knowing from the beginning of the play that Shakespeare's Othello is a tragedy, the various undoings of each character in Act IV while upsetting and difficult to observe, are not surprising. It is in this Act that the culminating tragedy is assembled- there is no turning back (perhaps a path that could lead to comedy) and Iago's horrible plans to strip Cassio of his honor, turn Othello both mad and against his wife and successfully paint Desdemona as unfaithful to her husband have all worked amazingly to his favor. No one suspects nor attempts to stop Iago and it appears that it is in Act IV that he has reached the level of status and importance and control that he yearned for in the first few lines of the play- and he doesn't even seem to fully recognize it yet.
In Act IV Othello is now in the throes of insanity (or so it seems to everyone other than our resident evil-trickster Iago) and is completely convinced that Desdemona is not only cheating on him with Cassio, but that he must kill her for her infidelity. Iago is offered the utmost respect from Othello, even though it is during their initial conversations in Act IV Scene I that Othello finally believes without a doubt that his wife has been unfaithful and he begins to lose all of his composure. Othello is clearly upset, dramatic and furious at his wife but manages to offer a rather pleasant composure towards Iago during all of this;
Iago: "Do it not with poison. Strangle her in her bed, even the bed she hath contaminated."
Othello: "Good, good, the justice of it pleases, very good."
Iago: "And for Cassio, let me be his undertaker. You shall hear more by midnight."
Othello: "Excellent good."
(Act IV Lines 197-202)
If one simply overheard this conversation between these two characters, by the general flow of the conversation I'm sure you would assume that Iago is the man with the power and ranks above Othello, rather than the other way around. It is Iago that is telling Othello what to do; it is Iago who is providing the information and the plans. Othello was introduced to the audience as the renowned General; he gives the orders and he has successfully managed to keep Venice safe from Turkish intruders. Now, it is Iago (who we met as Othello's sullen ensign, throwing a fit over not receiving a promotion) is giving the orders! Through his mastery of deceit and his luck (what with the handkerchief fiasco) the world of this play is revolving in his hands and he is the only one who knows (with, perhaps, the exception of Roderigo).
Despite knowing that Othello was a tragedy from the moment I picked up the play, I couldn't help but root for Othello and hope that he'd desperately be able to maintain both his sanity and the right sense of what is "true". It seems even more unfair that an underdog like Othello (as perhaps one of the few non-white men of such esteem in Shakespeare's time) would fall victim to a plot so sinister as the one Iago has woven. Now all the audience can do is sit and watch as the play unbearably unfolds into a tragic ending- surely filled with wrongful deaths and ignorance over the truth of Desdemona's fidelity.