How does betrayal affect the plot of Macbeth?
Betrayal drives the plot of the play. First, Macbeth betrays his king, friend, guest, and kinsman when he murders Duncan. Macbeth has always been loyal to the king, and he recently fought valiantly on two different fronts in order to secure Scotland against traitorous rebels and foreign invaders. However, once the Weird Sisters tell him that he'll become Thane of Cawdor and king, and then he actually learns that he's been named the Thane of Cawdor, his ambition consumes him and he eventually betrays his king and country.
Later, Macbeth betrays his former best friend, Banquo, ordering his murder as well as the murder of Banquo's son, Fleance, because the Weird Sisters told Banquo that his descendants would be kings. Macbeth regretted Duncan's murder, but by the time he betrays his best friend, his conscience is basically dead. Betrayal of those by whom one is trusted seems to be a sure way to become utterly morally corrupt.