Why does Macbeth kill Macduff's wife and children?
Macduff is one of the first thanes to turn against Macbeth, if not the first. When Macduff discovered King Duncan's body, Macbeth was present and was acting so strangely that Macduff suspected him of the assassination before anyone else did. Macduff flees to England to join Malcolm in raising an army to invade Scotland and place Malcolm on the throne. Macbeth wants revenge for this desertion, but more importantly he wants to set an example to dissuade others from following Macduff. Macbeth's biggest problem is that he is in danger of losing military support through desertion. There is nothing secretive about the murders of Macduff's wife and children; Macbeth wants everybody to know who was responsible and why. In the end it appears that Macbeth's terrorist tactics only made others flee to join the English army. At the beginning of Act 5, Scene 5, with the English forces approaching, he says:
Were they not forced with those that should be ours,/ We might have met them dareful, beard to beard,/ And beat them backward home.
He was able to surprise Macduff's castle guards and murder Macduff's family, but the other thanes would have been forewarned by Macbeth's bloody revenge and would have taken care to protect their families before they fled to England.