to gather the support of both the King of England and Malcolm (Duncan's son) in overthrowing Macbeth. So, that is his basic motivation as it relates to the story. As for the structure of the play, Macduff's departure to England is important for two reasons. First, with Macduff gone, his family is left vulnerable, thus Shakespeare has a perfect opportunity to show how truly despicable and cold-blooded Macbeth has become with the murder of Macduff's wife and children. There is a brief scene before their murder, in which Macduff's wife questions her husband's motivations for leaving them at such a dangerous time. This adds the audience's experience by giving them something else to consider when it comes to Macduff's true motives for leaving. Secondly, the meeting between Macduff and Macbeth allows the audience to see the truly noble motivations of Macduff and Malcolm. Malcolm's clever, yet untrue description of what kind of king he would be gives Macduff the opportunity to show the audience how much he truly wants what is best for Scotland. Both men realize that the other is truthful and noble in their intentions, which raises the level of suspense as the audience realizes that there is no way out for Macbeth; he has lost everyone and will lose everything soon enough.

After assassinating Banquo, Macbeth visits the witches who prophecied that he would become King of Scotland.  The witches prophecy to beware of Macduff, the thane of Fife.  Then they prophecy that he "need not fear any man of woman born."  Then they prophecy that his kingdom will not fall until Birnam Wood comes to the Royal Palace.  Despite the mixed message of the prophecies (A forest cannot move, All men are born of women aren't they?) Macbeth takes preventative measures, and sends assassins to kill Macduff and his family.  Macduff escaped, his family did not. 

Macduff flees to England, for his own safety and then joins Malcolm (the rightful heir to the Scottish throne) in a coup to overthrow the tyrant Macbeth.