From the start of Shakespeare's Othello,
Iago makes it very clear that he holds no love for the title character. In his opening argument with Roderigo, Iago says that his anger stems from the fact that Othello unfairly passed him over for promotion and made Michael Cassio his lieutenant, even though Cassio, unlike Iago, has no military field experience.
Although this is a good reason to be angry at Othello, Iago later reveals to the audience what he doesn't reveal to Roderigo. In a soliloquy at the end of Act I, Scene 3, Iago divulges perhaps the true nature of his hatred toward Othello:
I hate the Moor;
And it is thought abroad that 'twixt my sheets
He has done my office: I know not if't be true;
But I, for mere suspicion in that kind,
Will do as if for surety.
It appears that there are rumors that Othello has slept with Iago's wife, Emilia. Here, Iago says to himself — when no other characters are able to hear — that he doesn't care whether the rumors are true; he's going to proceed as if they are indubitably true.
Throughout the play, Iago's soliloquies disclose to the audience what he does not reveal to the other characters as his plans take shape. At the end of Act II, Scene 1, he reiterates the real motivation behind his plots against Othello:
I do suspect the lusty Moor
Hath leap'd into my seat: the thought whereof
Doth, like a poisonous mineral, gnaw my inwards;
And nothing can or shall content my soul
Till I am even'd with him, wife for wife.
But underlying all of Iago's excuses and justifications is a deep-seated racism, which is apparent just by looking at how he refers to Othello. When he isn't, in private, calling Othello a "Barbary horse," an "old black ram," or an "erring barbarian," he usually refers to him in public simply as "the Moor," identifying Othello not as an individual or even by his rank, but by his race. In today's terms, this would be like calling someone "the Jew" or "the Arab" all the time.
Such blatant racism wouldn't have been as shocking or disturbing to Shakespeare's audience, but modern performances of this great tragedy cannot avoid calling attention to the implications of Iago's prejudice.