"by making him uncapable of Othello's place - knocking out his brains"
Iago states that there could not be a man that would attempt to poison Othello's mind, Emilia reminds Iago that he had believed that she had slept with Othello previously. Desdemona suggests that whatever happens she will always love Othello.
Emilia and Desdemona exit to attend dinner, Roderigo arrives and accuses Iago of taking advantage of him and keeping jewels that he was supposed to have passed on to Desdemona for him. He threatens Iago if he finds out that this is infact true.
Iago says that he would not do such a thing, and promises Roderigo that soon Desdemona will be his. Finally, he plots for Roderigo to kill Cassio that same night so that he cannot have Desdemona, Cassio says that he needs more information before doing so.