Disturbed by the vision of Banquo's ghost, Macbeth, at the end of Act III resolves to visit the witches as he wants them to confirm his path. Hence, Act IV commences with the witches making potions . This visit spurs him onward towards his ultimate goal. He no longer distinguishes good from evil and his obsession is all-consuming. He is demanding of the witches as they are the courage that he otherwise lacks now that he does not have the support of Lady Macbeth. However, it is not sufficient for them to give him vague assurances - he wants answers
that will confirm the actions he has already taken ...
Maccbeth is so misguided by the visions that he feels no remorse at the killings which are mounting up.
He also does not allow for any other interpretation of the apparitions other than his own.
Macbeth's interpretation of the prophesies are such that he feels invincible and even justified. He feels that , as the witches prophesies already came true when he became Thane of Cawdor, he must be the one to ensure the culmination of everything they spoke of.
This supernatural soliciting
Cannot be ill, cannot be good
was enough for him to take matters into his own hands to ensure their resolution in his favor, regardless of the cost. After visiting the witches in Act IV, Macbeth is assured of victory as there are some things he knows for certain - man is born of woman and woods do not walk:
Macbeth shall never vanquished be until
Great Birnam Wood to high Dusinane Hill
Shall come against him.
As Act IV progresses, the audience, if it had any doubt of Macbeth's fatal flaw (his ambition), is left in no doubt that Macbeth has passed the point of no return when it
unflinchingly depicts the murder of an innocent child.
This provides the stark contrast in Macbeth - the innocents. They collectively add to the drama by being loving and providing a human element and even
an oasis in the mayhem
in a situation that otherwise would be overridden by the supernatural.
There is also the distinction that, in Shakespeare's day it was noble to defend your king and your family. So Malcolm and MacDuff will be justified in their destruction of Macbeth as he is now their enemy. Macbeth, having originally been a noble soldier, rewarded for valour, had no such reasons and was not defending his position - only his all-consuming ambition.
His fate is sealed.