Macbeth is a play of contrasts and conflict, especially between good and evil and appearance and reality. The witches comments of
fair is foul
are cemented by Macbeth himself
So foul and fair a day I have not seen (I iii 39)
There is a struggle by Macbeth between achieving his ambition and behaving in a good and moral manner. Macbeth is considered a tragic hero by most and antithesis is needed to create that balanced contrast.
and nothing is
But what is not. ( I iii 41)
He is uncertain about the witches' words and has an opportunity to discount them
This supernatural soliciting
Cannot be ill, cannot be good (I iii 141-142)
but it does not take long for Lady Macbeth to convince him of his purpose.
Lady Macbeth is the epitome of antithesis. She and Macbeth have a
genuine, if distorted, bond of love holding the two together
The reversal of roles and values - Lady Macbeth 'desexing' herself and being the driving force behind Macbeth to the point where she will completely remove any maternal instinct as she
..dashed out its brains..
The execution of the Thane of Cawdor, an acceptable occurrence in the day for acts of treason, reinforces the common thread that this play is a contradiction:
Nothing in his life
became him like the leaving of it.(I.iv.7-8)
Refer to the eNotes study guide and pages to see examples of the ambivalence and themes of Macbeth which reinforce the use of antithesis.