Macbeth wants Banquo dead because he worked hard to become king and wants his descendants to inherit the throne, but the witches prophesied that Banquo's sons would take the throne after Macbeth died. Banquo also suspected Macbeth of killing the king to take the throne, so Macbeth wants to get rid of him before he says anything to anyone.
Macbeth plans to have Banquo and his sons murdered by hiring convicted murderers to wait along the road that they will be traveling on and attack them. Banquo is murdered but the sons escape the attack. Macbeth is later confronted by Banquo's ghost at the dinner table.