Why does Macbeth kill Banquo?

Macbeth kills Banquo because he sees Banquo as another threat to the throne. In the Witches’ original prophecy, they proclaim that Macbeth will be king but that Banquo’s son and descendants will be the future kings, while Banquo will never be king himself. Macbeth, never fully understanding how the prophecy would manifest, once again takes matters into his own hands. Even though Banquo is his close comrade, Macbeth is now on a single-minded mission to protect himself and his position, and he kills Banquo to maintain the throne.