Iago not only wanted revenge on Othello, but as well on Cassio for stealing his job away from him. "If I can fasten buy one cup upon him, with which he hath drunk tonight already, he'll be as full of quarrel and offense as my young mistress' dog" Othello. Act II. iii. 49-52. Iago got Cassio so drunk that he got into a fight with another officer, and when Othello found out about this, he quickly fired him. Iago, befriending Cassio, told him to speak to Desdemona about getting his job back and this was done for a reason. Iago states his motives clearly. He intended to use Desdemona's righteousness against her. .
"For whiles this honest fool (Cassio) Plies for Desdemona to repair his fortune, And she for him pleads strongly to the Moor, I'll pour this pestilence into his ear: That she repeals him for her body's lust; And by how much she strives to do him good, She shall undo her credit with the Moor. So will I turn her virtue into pitch, And out of her own goodness make the net That shall enmesh them all." Othello Act II. iii. 373-382.
Desdemona's handkerchief plays a large role in Iago's web of destruction. Upon losing the handkerchief which Othello has given her, Emilia finds it and gives it to Iago who then places it in Cassio's room, and then tells the Moor that Desdemona had given it to him. This was one of the many so-called proofs to come of how Desdemona was sleeping with Cassio. Iago outright continuously lies to Othello, and Othello like a fool, believes every word he says to him. .
"In sleep I heard him say "Sweet Desdemona, Let us be wary, let us hide our loves. And then, sir, would he gripe and wring my hand, Cry "O sweet creature!" then kiss me hard, As if he plucked up kisses by the roots That grew upon my lips; then laid his leg Over my thigh, and sighed, and kissed, and then Cried "Cursed fate that gave thee to the Moor!" Othello. Act III. iii. 475-482.
Iago continued to torment Othello with vivid descriptions of Desdemona's alleged sexual affair with Cassio until Othello blacks out.