Why doesn't Macbeth kill Macduff? How is this significant to the play?
In Act IV, Scene 1, the apparitions warn Macbeth to be careful of Macduff, but also maintain that "none of woman born/ Shall harm Macbeth."
Macbeth is happy to hear this, but soon decides he will rid himself of Macduff anyway, just to ensure his chances of maintaining his hold on power. He soon discovers Macduff has fled England, and is miffed he didn't act sooner. To ensure no heirs follow Macduff, he orders Macduff's wife and children to be killed.
The castle of Macduff I will surprise,
Seize upon Fife, give to th' edge o' th' sword
His wife, his babes, and all unfortunate souls
That trace him in his line.