In Macbeth, explain how the vision of eight kings in a line, with a bloody Banquo, could have helped Macbeth realize the truth about the prophecies?

Macbeth should have realised that the line of kings from
Banquo's son onwards would seem perpetual. Those members of the audience with a
knowledge of history would be aware that the monarch at the time the play was performed,
James I, was directly descended from Banquo's line. In this way Shakespeare is utilising
some (but certainly not all) of the historical events upon which the play is loosely
based. Also, as Macbeth had disrupted the Natural Order and the Divine Right of Kings
which were strong beliefs of the time, it was to be expected that his hold on the throne
would be brief, and his audacious plan to subvert God's Will would be
punished.

In the light of this vision, the prophesies as
given to Banquo in Act I scene iii become clearer in their
meaning-

Lesser than Macbeth, and
greater.

Thou shalt get kings,
though thou be none.

Banquo himself
does not live to be king, but his bloodline does.