What is the significance of Macduff's comments in Macbeth, Act II?
This is the first time that Macduff speaks in the play (he appears in I/6, with no lines), and it establishes several key facts about his character: he is loyal, impulsive, and blunt to the point of tactlessness.
After his conversation with the gate-porter at the beginning of Act II scene 3, during which he banters ironically with this figure who has invoked Hell, Macduff goes to wake the king and thus becomes the first to "officially" learn of the murder. His horror is open and violent:
Banquo and Donalbain! Malcolm! awake!
Shake off this downy sleep, death's counterfeit,
And look on death itself! up, up, and see
The great doom's image! Malcolm! Banquo!
As from your graves rise up....
There is a pointed contrast between the violence of his rhetoric and the behavior of Macbeth, whose speeches are much more consciously contrived but who still manages to "lose control" and kill the king's servants, the alleged murderers. The contrast is not lost on Macduff, whose question to Macbeth about his reasons for this action is curt and immediate.
His bluntness is underlined in the next scene, Act II scene 4, when Macduff, although he repeats the "official" story of the murder, states that he will not attend Macbeth's coronation. He adds,
Well, may you see things well done there,--adieu!--
Lest our old robes sit easier than our new!
He is clearly sceptical, at the very least, of Macbeth's fitness to rule, and unable to hide his feelings behind a polite cloak.
Macduff's comments in act 2 hint at his suspicion of Macbeth's involvement in the death of King Duncan.
After learning of King Duncan's murder, Macduff is skeptical about the guards' having committed the "Most sacrilegious murder" of Duncan (2.3.89). For one thing, when Macbeth purportedly admits that he killed Duncan's guards out of the "expedition of [his] violent love," Macduff is dubious (2.3.85). He asks Macbeth, "Wherefore did you so?" (2.3.86). Macbeth responds to Macduff not with logic, but with an emotional appeal. He asks Macduff who could refrain from killing these guards after seeing their daggers "breeched with gore" (2.3.95) if he loved the king and had the courage "to make [his] love known?" (18.104.22.168.97)
Then, when Banquo expresses his suspicions of "treasonous malice," Macduff concurs. In the next scene, Macduff is asked by Ross if he knows who has killed King Duncan. Macduff replies not by saying that it was Duncan's guards, but by saying, "Those that Macbeth has slain," (2.4.21) a phrase that hints at some suspicion of Macbeth. Further, the fact that Macduff has doubts about Macbeth's integrity is evinced in his refusal to attend the coronation of Macbeth.