Because he thought and other people thought that they are not equal to each other! hope this helps! ;3
The event in Act IV that foreshadows Macbeth eventual downfall is when Lady Macduff and her children are murdered. So Macduff finds out about Macbeth and he vowed to kill Macbeth himself. Macbeth encounters the witches who cryptically present three sprites that give him prophetic warnings. When he asks them for elaborations they preform a ritual and vanish.
