Iago expresses his plan and purpose in a soliloquy at the end of Act 1, Scene 3. He plans to get Cassio's position as Othello's lieutenant by making Othello jealous of the handsome, flirtatious younger man, and at the same time he plans to get revenge against Othello by making him jealous of Desdemona. Iago describes his purpose as "double knavery." He can gain a promotion for himself by undermining Cassio's position, while at the same time he can spoil Othello's new marital happiness by making the Moor jealous of his beautiful wife. At this point in the play it is not suggested that Iago is thinking of getting Othello to murder Desdemona or even getting him to divorce her. He simply wants to create jealousy and suspicion. Iago also states in this soliloquy that he intends to get as much money as possible out of Roderigo, who is foolishly infatuated with Desdemona.
Arguably Shakespeare's most heinous villain, the duplicitous Iago plans to avenge himself against Othello for being passed over for promotion. He decides to exploit Cassio in this plan.
In his soliloquy at the end of Act I, Scene 3, Iago decides to use Cassio to hurt Othello. He plans to incite Othello's jealousy by intimating that Desdemona and Cassio are having an affair. Because Cassio is good-looking, Iago reasons, Othello will easily believe that Desdemona has been seduced by him. Iago also plans to exploit Othello's ingenuousness:
The Moor is of a free and open nature
That thinks men honest that but seem to be so,
And will as tenderly be led by the nose
As asses are. (1.3.379-382)
Interestingly, in his aggressive soliloquy Iago demonstrates the flaw that has caused Othello to not promote him and to give Cassio the position instead. That is, Iago is incapable of being anything but bellicose, while Cassio is diplomatic and is aware of the limits of war.
According to renowned Shakespearean critic Harold Bloom, in Iago Othello sees a man who cannot stop being at war, and for this reason, Iago could not replace Othello if the general were to be killed or wounded. The skilled warrior Othello understands that there is a time for war and a time for peace.
Additional Source: Bloom, Harold. Shakespeare: the invention of the Human. London: Fourth Estate, 1999. Print.