In Shakespeare's "Othello," Iago carefully and masterfully entraps Othello into believing that his wife, Desdemona, is having an affair with Cassio. He does this through a series of suggestions and hesitations that entice and implant images into Othello's head that lead him to his own demise. More importantly, Iago gives Othello the motive to murder his own innocent wife Desdemona, satisfying Iago's immense appetite for revenge.
The motive for Iago's devious plan is initially made clear in the first of three major soliloquies, in which he proclaims Othello has had an affair with his wife, Emilia: "And it is thought abroad that t'wixt my sheets/ He's done my office" (I.iii.381-383). The irony behind this line is where he continues: "I know not if't be true/ But I, for mere suspicion in that kind; / Will do as if for surety"(I.iii.383-385). Iago is so exceedingly paranoid and insane that he will go far as murdering, and deluding even a general into murdering his wife.
Iago simultaneously conducts a devious plan to obtain Cassio's position as lieutenant, using Desdemona's prime weakness; her naivety. He disgraces Cassio by intoxicating him enough so he strikes Roderigo. Othello then discharges Cassio of his Lieutenancy when he says: "Cassio, I love thee,/ But nevermore be officer of mine" (II.iii.242-244). It was therefore understandable that he would fall to the mercy of Iago, completely oblivious to the inevitable effects. Iago reveals his plan to the reader in his third soliloquy when he states:
His soul is so unfettered to her love,
That she may make, unmake, do what she list,
even as her appetite shall play the god
With his weak function...
And she for him pleads strongingly to the Moore,
I'll pour this pestilence into his ear:
That she repels him for her body's lust,
And by how much she strives to do him good,
She shall undo her her credit with the Moor...