In Macbeth, when Macduff and Malcolm meet in England, in Act IV, scene iii, they speak of how their "poor" country is reeling under Macbeth's "Great tyranny" (line33). Malcolm (the future king) is unsure of Macduff's allegiance, however and, although Macduff speaks of fighting to regain, as he says, "Our down-fallen birthdom" (line 4), Malcolm cannot help but question his intentions.
Macduff wants to act without delay because with each passing day there are more atrocities as "New widows howl, new orphans cry..." (line 5) suggesting that men are dying mercilessly under Macbeth's reign. The men speak of "this tyrant, whose sole name blisters our tongue" (line 12) as Macbeth, whom Malcolm had considered an honest man certainly loved by his men (including, he points out, Macduff), continues to cause great strife in Scotland.
Malcolm tests Macduff, indicating his mistrust and suggesting that Macduff may in fact be loyal to Macbeth. Perhaps Macduff is only in England trying to get Malcolm to return to Scotland in order to "appease an angry god" (line 17), meaning that Macduff may be part of Macbeth's plan to destroy the rule of Scotland by getting rid of the true heir.
Macduff is shocked that Malcolm would believe this of him but Malcolm reminds him how appearances often belie the truth. Even angels fall and Macduff has left his own family in peril. However, such is Macduff's loyalty that he knows that while Scotland suffers, he must do what he can to return Malcolm to his rightful place as king of Scotland. They agree that, as Malcolm points out, Scotland "sinks beneath the yoke; It weeps, it bleeds..." (line 40). However, still testing Macduff, Malcolm suggests that it is possible that, under Macbeth, the rule is better than if he, Malcolm, rules Scotland. He suggests that perhaps he is a worse ruler than Macbeth. Macduff despairs at such words and vows to never return to Scotland if Malcolm does indeed "blaspheme his breed" (108).
Upon this, Malcolm realizes that Macduff is, in fact, loyal to him and can be trusted against "Devilish Macbeth," whom they are now determined to conquer, especially when Ross confirms what they say about Macbeth's reign, that Scotland is a "poor country, almost afraid to know itself..."(165).