In Scene 4, Macbeth asks his wife, "What say'st thou that Macduff denies his person / At our great bidding?" Macduff is getting ready to flee to England. Macbeth says, "There's not a one of them but in his house / I keep a servant fee'd." He is aware of the gathering opposition and feels he has to spy on everyone.
Then in Scene 6, Lennox, who was once so young and naive, expresses the general belief that Macbeth murdered Duncan and Banquo and would have found a means to murder Duncan's sons Malcolm and Donalbain if they hadn't fled Scotland.
Then the Lord to whom Lennox has been speaking advises him that Macduff has fled to England and is helping to raise an army to attack Macbeth. Lennox concludes this scene with the hope "that a swift blessing / May soon return to this our suffering country / Under a hand accursed."
Macduff's defection from Macbeth and influence with the English King Edward are emphasized because it is Macduff who will ultimately kill Macbeth on the battlefield. Shakespeare, presenting his drama on a small stage, wishes to personalize the conflict, since he cannot show entire armies engaged in battle.