In act 4, scene 3, Malcolm determines to put Macduff's trustworthiness to the test. Thus, in this scene, he admits to possessing a series of vices that would make him a terrible ruler. He claims to be overly lustful, then extremely greedy, before finally claiming to possess, in Shakespeare's words, no "king-becoming graces." As Malcolm states:
I have no relish of them; but abound
In the division of each several crime,
Acting it many ways. Nay, had I power, I should
Pour the sweet milk of concord into hell,
Uproar the universal peace, confound
All unity on the earth.
At this point, Macduff falls into despair, rejecting Malcolm as unfit for ruling Scotland. As it happens, however, this reaction is precisely what Malcolm was looking for: it is Macduff's rejection of Malcolm that proves his trustworthy nature.
Remember, Macbeth had previously usurped and murdered Malcolm's father, Duncan, and Malcolm is planning to win back the throne through armed conflict. In such a context, Malcolm would have been deeply worried about Macbeth sending agents among his forces. By rejecting Malcolm, however, Macduff inadvertently confirms that he is not a spy, and lacks these ulterior motives.
Perhaps even more fundamentally, however, Macduff's own despairing rejection of Malcolm establishes Macduff's own character as someone of principle. As he establishes in this encounter, he will not follow Malcolm blindly and without criticism, but rather is driven by a higher sense of virtue and love for his country. Even beyond the threat of Macbeth's spies and agents, Malcolm must also wrestle with Macbeth's very example as a trusted vassal who betrayed and murdered his king for his own political gain.