Why does Macbeth kill Banquo?

After they become King and Queen of Scotland, Macbeth and his wife remember the prediction that Banquo's sons shall reign, so they determine to kill Banquo and Fleance, his son, and invite them to supper. Macbeth sends murderers to kill them on their way to the supper. Banquo is stabbed, but Fleance escapes.