In Act 4, Scene 3 Malcolm tests Macduff’s loyalty.
Malcolm was King Duncan’s oldest son, but still quite young. When Macbeth killed his father, he fled to England. He did not just sit back and pout though. Instead, he raised an army. Since there was betrayal everywhere, he needed to make sure that Macduff was completely loyal to him. In line 20, Macduff says he is not treacherous.
It is myself I mean, in whom I know
All the particulars of vice so grafted
That, when they shall be open'd,
(Act 4, scene 3, p. 68)
Malcolm tells Macduff that compared to him, Macbeth will seem as white as snow. He talks about whether he is fit to govern, and even tells Macduff that the daughters of the kingdom will not be safe from his lust.
Macduff stays true. He is loyal and honest, and stands by Malcolm. Malcolm is convinced that he is loyal, and he will be a good lieutenant in the war to regain the throne from the bloody tyrant Macbeth.