Also signficant here is the way that Macduff acts as a foil for Macbeth (and Lady Macbeth, too, perhaps). His first inclination upon hearing of the death of his family is to grieve. "All my pretty ones? / Did you say all?" he moans. When Malcolm says "Dispute it like a man," meaning go and fight Macbeth, Macduff says "I shall do so;/but I must also feel it as a man" (4.3,216-221). Here we have a new definition of manhood not yet seen in the play. Indeed, Lady Macbeth at the beginning worries that her husband is "too full the milk of human kindness" and would herself dash her babe to the ground in order to carry out the murder of Dunca. Macduff is both warrior and father and husband, a man who can love as well as fight.
Act IV, Scene 3 begins with Macduff and Malcolm in England. Malcolm tests Macduff's sincerity by telling Macduff he has done far worse things than Macbeth, but Macduff responds by saying Malcolm is the rightful heir to the throne. Shakespeare uses this to show that Malcolm is a good man and will be a good king for Scotland. When Ross tells Macduff that Macbeth has killed his family, Macduff is even more determined to seek revenge against Macbeth, and he and Malcolm vow that this will be the last evil act of Macbeth. The conversation between Macduff and Malcolm also reinforces a theme of the play as well: that a true king is one who is motivated by his love for his country, and Macbeth's is certainly not a true kingship because it has no moral legitimacy.