When Macduff visits Malcolm in England in act 4, scene 3, Malcolm is suspicious of Macduff's motives and desires to know if he was sent by Macbeth. Malcolm proceeds to portray himself as a person with many vices, who is not fit to rule Scotland. Macduff is initially astonished and heartbroken by Malcolm's false confession and addresses him as the rightful heir to the Scottish throne. After Macduff comments that Macbeth should be unseated at all costs, Malcolm is convinced that he is sincere and reveals that everything he said was a lie. Malcolm purposely misrepresented himself to avoid being betrayed by Macduff. Malcolm's father was the recent victim of betrayal and he is attempting to avoid the same fate. Malcolm knows that he cannot trust anyone at the moment and exercises caution by questioning Macduff's character and intentions. Malcolm is also aware that Macbeth is willing to send assassins after him and bribe presumably trusted officials and acquiesces to set him up. Overall, Malcolm purposely misrepresents himself to test Macduff's loyalty and sincerity. Macduff passes Malcolm's test by demonstrating his passion to destroy Macbeth and restore him to his rightful position as king.
The play is about betrayal and trust. We see that Duncan made a fatal mistake in trusting Macbeth. Malcolm does not want to make the same mistake in trusting Macduff, who wants him to return to Scotland. Fearing a trap (Remember Braveheart?), Malcolm must test Macduff's loyalty. After all, Macduff's presence in England is somewhat suspicious. Malcolm wonders how Macduff could leave his wife and children to come to England. Perhaps Macbeth sent him and has guaranteed protection for them. If Malcolm returns to Scotland with Macduff, how can he be sure that some sort of ambush is not awaiting him. He knows that Macbeth would like to see him dead, and Macduff could be working for Macbeth. So, he lies to Macduff and tells him that he is the worst kind of man: greedy, lustful, devoid of kingly virtues. If Macduff still wants him to return to Scotland, then Malcolm knows that it is a trap. But when Macduff hangs his head and walks away in frustration, Malcolm knows that Macduff is sincere in his desire to have the rightful monarch on the throne of Scotland.