In asking Desdemona for help in restoring Othello’s faith in Cassio, Iago has drawn each of the players together for his own cruel drama.
Iago is intent on the downfall of those around him, including Cassio, Othello and Desdemona. He wishes Cassio to be out of favour with Othello as he believes that Cassio has taken the position as Othello’s lieutenant which should have been his
He, in good time, must his lieutenant be,
And I—God bless the mark!—his Moorship's ancient.
He is therefore angry with Cassio and Othello.
Were I the Moor, I would not be Iago.
In following him, I follow but myself;
Iago’s plan for revenge involves making Othello mad with jealousy in implying that Othello’s wife, Desdemona, is being unfaithful to him with Cassio. Iago believes that Othello has slept with his wife, Emilia, so such revenge, he sees, is appropriate. He is happy to condemn Desdemona, and turn her virtue ‘black as pitch’ in the process. He has no respect for her as he has no respect for women in general.