In Act III, Scene I we learn that Macbeth has hired murderers to kill Banquo and his son. Macbeth does this because he realizes that Banquo poses a threat to Macbeth's rise in power. At the same time, Banquo loses trust in Macbeth but decides to fake his loyalty for safety. He also wonders if the witches' prophesy concerning his sons will actually come true, which puts them directly at odds with Macbeth.
Banquo is murdered in Act III, Scene III. However Banquo's son escapes. When the murderers tell Macbeth that the plan was not completely successful Macbeth realizes that Fleance still is a threat but is happy that no other children will be born of Banquo.
Banquo dies in Act III