Act four, scene 1

This is the cauldron scene. As you will recall, Macbeth had decided to go and meet the witches to demand more answers. The witches show him 3 apparitions which give him three messages.
The first apparition tells him to beware of Macduff. However, the second apparition tells him that no one born of a woman may harm him. Thus, Macbeth sees this apparition as cancelling out the second. He feels confident that Macduff cannot harm him; however, as his ambition and desire to have no worry or fear overcome him, he decides to have Macduff killed anyway, just to be sure. The next apparition is even stranger; Macbeth is told that he will not be vanquished until the great Birnam wood comes to Dunsinane hill. In other words, Macbeth feels he is completely safe, as it is impossible for a forest to walk to the castle.

However, Macbeth insists on having one more question answered: will Banquo's son ever reign? The witches tell him yes, they will, and Macbeth sees a vision of a line of kings that stretches on for a very long time. He is extremely angry at this vision and curses the witches. He then finds out that Macduff has fled and determines to go and have his family and household murdered anyway. At this point, Macbeth's ambition and evil acts have completely overcome him--he ignores how wicked an act it would be because he wants to ensure that no one can harm him. He is causing much suffering and soon the forces of good will be rallying against him.