What lines from Macbeth (besides the banquet scene where he sees Banquo's ghost) suggest he is mentally unstable?
In Act IV, Scene 1 of Shakespeare's Macbeth, Macbeth once again encounters the witches. This time he actually seeks them out, hoping for some information about his future. Prior to this scene we see Hecate, the goddess of sorcery, berate the witches for their previous behavior toward Macbeth. She believes that they have been too easy on him. So Hecate and the witches plan to make his life more difficult in the future.
In this scene Macbeth is asking several things of the witches. The fact that he trusts them at all points to his growing instability. He wants to know if he needs to fear Macduff. The witches answer cryptically in terms that he thinks he understands but really does not. Then he asks about Banquo’s descendants, wanting to know if they will ever take the throne of Scotland. The witches respond with a line of visions of future kings, all related to Banquo. The sight of this parade seems to throw Macbeth over the edge:
Thou are too like the spirit of Banquo. Down!
Thy crown does sear mine eyelids. And thy hair,
Thou other gold-bound brow, is lke the first.
A third is like the former. Filthy hags!
Why do you show me this? A fourth! Start, eyes!
The sight is too much for Macbeth. After this he vows to take swift action in the future to protect himself, starting with the slaughter of Macduff’s family.