Act 4: False Security and Revenge
Macbeth returns to the witches for more guidance, and they summon three apparitions with cryptic warnings. They tell him to fear Macduff, that no one "born of woman" can harm him, and that he can't be defeated until Birnam Wood comes to Dunsinane Hill.
These prophecies make Macbeth feel invincible, but he's still worried about Macduff. When he learns that Macduff has fled to England, Macbeth orders the brutal murder of Macduff's wife and children - an act of pure evil that shows how far he's fallen.
In England, Malcolm tests Macduff's loyalty before revealing they're building an army. When Macduff learns about his family's murder, he vows to kill Macbeth himself - this is now personal, not just political.
Think about this: The apparitions' prophecies seem to protect Macbeth, but they're actually setting him up for his downfall - classic dramatic irony!