hatred forOthello, and indeed all of his fellow men. His use of verse when speaking to Othello and others symbolizes the power he has over himself, others and the situation at hand. When he is relaxed in his manner (as he occasionally appears with Roderigo) Iago can forego the order of the verse form.

I have told thee
often, and I retell thee again and again, I hate the Moor. My
cause is hearted; thine hath no less reason. Let us be
conjunctive in our revenge against him. If thou canst
cuckold him, thou dost thyself a pleasure, me a sport..

Iago, more than any other Shakespeare character, is a master of control. He makes a conscious decision to appear the loyal and ‘honest’ ensign to Othello, and yet he tells Roderigo, and the audience, that he has a seething hatred and resentment within himself. He uses prose when Othello loses control and he is no longer obeying form and convention. As Othello falls into a fit of jealousy in Act IV  scene I, Iago revels in the scene-

 Work on,
My medicine, work! Thus credulous fools are caught:
And many worthy and chaste dames even thus,
All guiltless, meet reproach.

 Iago maintains his control when at the end of the play he returns to the verse form as he resolutely refuses to explain his actions –

 Demand me nothing; what you know: you know.
From this time forth I never will speak word.