What does Malcolm tell Macduff he wants to become?

In Act IV, Scene III, Malcolm and Macduff are in England discussing the plight of their country and how evil Macbeth has become. Malcolm then starts to suggest that, as bad as Macbeth is, he will be even worse if he were to eventually become king. He goes on a tirade about all of his own vices. He tells Macduff that his own evil is limitless. He adds that his own greed is also boundless. Upon becoming king, he says he will take the lands and jewels of his nobles. His greed is so great, so he says, that taking from his people will only increase his lust for power and greed. He says that if he becomes king, he will "uproar the universal peace." In other words, he says he will become a terrible tyrant of a king. But he never says that he "wants" to become such a king. 

Macduff counters by saying that he does not believe Malcolm will become this kind of ruler. Malcolm agrees and says all of these terrible things he has been saying about himself are not true. Malcolm was only testing Macduff's loyalty.