Macbeth, in Scene 1.3: A witch says to Banquo, "thou shalt get kings, though thou be none; So all hail, Macbeth and Banquo!" In what way will this prediction be an impediment to Macbeth's ambition?

"Lesser than Macbeth and greater," "Not so happy, yet much happier," and a man who "shall get kings, though [he] be none" (63-65).

This prophecy alludes to the fact that although Banquo will never be king, they (his sons) will in fact sit on the throne. This implies that Macbeth's heirs will not..... thus, it makes Macbeth believe that Banquo is another impediment to his aspirations.