In Shakespeare's play Macbeth, why is Macbeth so upset to see the vision of Banquo and the eight kings?

When Macbeth goes to the witches to hear more prophecies later in the play, the witches conjure up three apparitions -- one says no man born of a woman can harm Macbeth, another says to beware Macduff, and a third says Macbeth's power will not be lost until the forest marches up the hill -- along with the vision of Banquo and the mirrored line of kings. Macbeth is satisfied with the apparitions, as he reads them in the most generous way for himself. He thinks they secure his power, and he admits he already knows to be wary of Macduff. The vision of Banquo and the line of kings, however, causes Macbeth to lash out at the witches. The reason this upsets Macbeth is that it recalls the prophecy given in Act I: That Macbeth would be King and Banquo would not, but Banquo would "get kings," as in "beget," or be the father to kings. The vision here confirms that Banquo will, indeed, be the father to a long line of at least eight kings; his legacy will continue for generations. Macbeth has tried to circumvent this prediction by having Banquo and his son Fleance killed, but Fleance escaped, so this is already a sore subject for Macbeth. Further, Macbeth has no sons of his own to carry his legacy into the future. Macbeth is beginning to see the meaning of the original statement made by the witches that Banquo would be "lesser than Macbeth but greater." Macbeth is furious to think that all of the crimes he has committed will do nothing more than ensure Banquo's and his family's legacy.