In Act IV, Scene iii, Malcolm and Macduff are in England. Malcolm is a little unsure of whom he can trust, so he questions Macduff's loyalty and then he uses reverse psychology to make sure Macduff is on his side.
First, Malcolm asks Macduff why he left his wife and child so quickly. Macduff responds that he is not "the villain that thou think'st" (IV.iii.40).
Malcolm then says he would be a worse king than Macbeth. Macduff responds by saying that no one could be worse than Macbeth. Malcolm continues to criticize himself. He says he will be a bad king, that there is no bottom to his desires ("voluptuousness"), and that in his greed, he would take all of the nobles' lands and jewels. Macduff has heard enough and says:
These evils thou repeat'st upon thyself
Have banish'd me from Scotland. O my breast,
Thy hope ends here! (IV.iii.126-28)
Finally, Malcolm reveals that he had been criticizing himself to see if Macduff would disagree with him. Now assured that he has Macduff's loyalty, Malcolm takes back his self-criticisms:
Unspeak mine own detraction; here abjure
The taints and blames I laid upon myself, (IV.iii.138-39)