How do we know that in Act 4 of Macbeth, Macbeth's second visit to the witches leads to his downfall?

In Act 4, it is apparent that Macbeth's second visit to the
witches leads to his downfall because Macbeth tries to control fate.  He commands the witches to
let their masters speak for them, so the witches allow Macbeth to see the apparitions.  After the
first apparition reveals that he should beware Macduff, Macbeth tries to ask questions to get
more information.  The witches, however, tell Macbeth that he can only listen, not speak and
command more answers.  But Macbeth does not take this to heart, and he tries to ask more
questions rather than simply listen to the information that is given to him.  When the third
apparition departs, Macbeth again demands that the witches answer his question about Banquo's
sons becoming kings.  This display of arrogance suggests that Macbeth will soon meet his end. 
The witches then show him the line of kings who are all kin to Banquo, and one of the witches
asks Macbeth why he looks so surprised.  She tells him that of course he knew this from the
beginning.  But Macbeth has thought that he could change the course of fate by murdering Banquo. 
This confirmation foreshadows the downfall of Macbeth.