Malcolm tests Macduff’s loyalty to Scotland rather than to Macbeth by pretending to be a terrible person who would make an even worse king than Macbeth has been. Malcolm believes that if Macduff is loyal to Macbeth and is only trying to lure Malcolm back to Scotland so that he can be assassinated, Macduff will say that he wants Malcolm to return and claim the throne no matter what. However, if Macduff is loyal to Scotland, then he will not want Malcolm to return and be the king after Malcolm tells Macduff how awful a human being Malcolm is.
Malcolm says that “there’s no bottom, none, / In [his] voluptuousness,” and he says that he is so lustful that there are not enough women in Scotland to satisfy him (4.3.73–74). Macduff assures him that Malcolm can “Convey [his] pleasures in a spacious plenty” as the king, even though being so lustful is not a great quality to have in a leader (4.3.85). At this point, Malcolm says that he is also beset by “stanchless avarice,” and that, were he king, he would take all wealth from his nobles and drain the wealth of the kingdom with his greed (4.3.93). Macduff assures Malcolm that there’s plenty of wealth in Scotland to satisfy him, though greed is an even worse quality than lust.
Finally, Malcolm claims that he has “none” of the “king-becoming graces” and that he would rather “confound / All unity on earth” than behave with justice or courage (4.3.107, 4.3.115–116). It is at this point that Macduff declares that not only is Malcolm unfit to govern Scotland, but he is also unfit to live on the earth. This convinces Malcolm that Macduff is not loyal to Macbeth, who has “sought to win [Malcolm] / Into his power” and lure him back to kill him (4.3.137–138). Malcolm consents to return to Scotland with Macduff.