Shakespeare’s Othello is a play about the jealousy of one man, not Othello, but Iago. It is Iago’s jealousy–of anyone who gets anything that seems better than that which he gets himself–which is the driving force of the play. It is Iago’s jealousy which enables him to provoke the same feeling in others, to use them to his own advantage, or at least to their disadvantage.
Iago expresses his sentiments at times throughout the play. One of the passages where it to me becomes most obvious is I. ii. 382-394, and I will use this passage as my point of departure. At the start of this passage Iago has just told Roderigo to put money in his purse. When Roderigo has gone off, Iago explains to the audience that of course he is only socialising with such a fool to be able to use him “for sport and profit” (I. iii. 385). The emotion Iago is able to exploit in Roderigo, is Roderiogo’s jealousy towards anyone who so much as touches Desdemona.
At this point in the play, the sentiment seems overloaded with racism, but it later becomes clear that Roderigo is just as easily incented against Cassio, which should indicate that it is pure (or at least almost pure) jealousy he feels. Next Iago states the fact, of which by now the reader is largely aware, that is, that he hates “the Moor”. The Arden notes quote Heilman: “the hate is prior, and a motive is then discovered” (note to line 386), concerning Iago’s elaboration of this hate.
I should say that an easily statable (is that a word? ) motive is discovered, but that he has already given us motives for the “prior hate” in earlier statements, but that these have been less “spellt-out”. The first motive is Iago’s general dislike of anyone who gets more highly rewarded than him, not regarding their deserts. The second is the fact that Othello has made Cassio his lieutenant instead of him, a slight he is not likely to ever forgive (an which provides him with ample jealousy towards Cassio for later use).
That this slight is upmost in his mind is obvious from the way the statement “Cassio is a proper man” (391) follows so seemingly unconnectedly upon the the reasoning of how to get at Othello. In using Cassio Iago is (or rather thinks he will be) able to eat his cake and have it too, to “plume up [his] will / In double knavery”. The reason Iago is such a successful schemer, at least through most of the play, is the ability his own jealousy provides him with of recognising the potential for the same feeling in others, and to exploit it to gain his own ends.
Moreover his jealousy seems to be his only fault, he is not dishonest in other respects, we have no reason to believe that he is anything but valiant in the battlefield, nor that he is unfaithful to his wife. And since this jealousy is generally unstated to anyone but the audience (he tells Roderigo that he hates Othello, but Roderigo is not likely to tell anyone else), his fellow characters have really no reason to distrust him, and thus he is able the better to use them.