Act 3: Paranoia Takes Hold
Now that Macbeth's king, you'd think he'd be satisfied - but power only makes him more paranoid. Act 3 shows how guilt and fear transform him from a reluctant murderer into a ruthless tyrant who sees enemies everywhere.
Banquo becomes Macbeth's biggest worry because he witnessed the witches' prophecies. Rather than risk his friend's suspicions, Macbeth hires murderers to kill both Banquo and his son Fleance. The plan half-succeeds - Banquo dies, but Fleance escapes, meaning the prophecy about Banquo's descendants becoming kings could still come true.
The banquet scene is pure psychological horror. Banquo's ghost appears to Macbeth (though no one else can see it), causing him to have a complete breakdown in front of his guests. Lady Macbeth tries to cover for him, but it's clear their marriage is cracking under the pressure.
Both Macbeth and Lady Macbeth are suffering from sleeplessness and nightmares. The crown they fought so hard to win is making them miserable. By the act's end, Lennox and other lords openly suspect Macbeth of murder, whilst Malcolm gathers an army in England.
Key Point: The ghost scene works whether you believe it's real or just Macbeth's guilty conscience - either way, it shows how completely the murders have destroyed his mental state.