In the first scene of the play Iago is convincing Roderigo that he too hates Othello. The reason Iago gives is that he himself deserves the job of lieutenant which Othello gave instead to Cassio.
In the soliloquy he gives another reason for his anger and hatred:
'it is thought abroad that 'twixt my sheets
He has done my office'.
Iago has heard a rumour that Othello has slept with his wife and he siezes on this to justify his hatred.
Also in the soliloquy, possibly still thinking about the rumour regarding his wife and Othello, Iago decides he will 'abuse Othello's ear' that Cassio and Desdemona are too friendly. The use of the word 'abuse' shows he knows how horrible it will be for Othello to hear such allegations about his wife. This is the first stage of his plan to destroy Othello.