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.