Suspicion and confusion are the order of the day after the death of Duncan and Macbeth intends that this confusion should ensure his own safety. "Fair is foul and foul is fair" is reinforced.
Macduff visits Malcolm who has fled to England to escape being blamed for his father's murder. Macduff is anxious to restore order in Scotland and place the rightful heir on the throne. Macbeth
must be unseated at all cost.
Malcolm is unsure of Macduff's allegiance and he berates himself claiming that "black Macbeth Will seem as pure as snow" ( IV.iii.51) suggesting that he, Malcolm has committed far worse crimes than Macbeth could ever contemplate.
Fortunately, Macduff does not believe him and it is obvious to him that Macbeth is the "devil More damned in evils..."
To be sure that Macduff has not been sent by Macbeth, Malcolm continues, " Better Macbeth than such as one to reign". (Iv.iii.65). Macduff is steadfast but does begin to wane in his faith in Malcolm- "O Scotland!Scotland" whereupon Malcolm wonders whether Macduff might take up the crown. Only when Malcolm sees Macduff's utter despair "When shalt thou (Scotland) see thy wholesome days again" and when Macduff thinks that Malcolm "by his own interdiction stands accursed"
does Malcolm admit to Macduff that he was testing his loyalty.
Macduff is mostly confused " Tis hard to reconcile" (IV.iii.139) but is relieved that Malcolm is indeed a man of honor whom he can trust and support.