This scene opens with a soliloquy in which Macbeth is tormented by his conscience as he contemplates killing Duncan. Macbeth uses such phrases as "poisoned chalice," "bloody instructions," and "deep damnation," with the effect being that the audience sees that Macbeth fully contemplates that what he is about to do is wrong, and that he is deeply conflicted about it. His anguish is contrasted with his wife, who enters the room, and, upon finding out that he no longer plans to go through with the murder, challenges his masculinity by saying that when he initially planned to do it, he was a man. Now that he was seemingly ready to back out of the scheme, she views him as a coward. Lady Macbeth then utters one of the most chilling lines from all of Shakespeare's tragedies when she castigates him for failing to live up to what he had sworn to do:
I have given suck, and know
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this.
So the audience sees a Macbeth persuaded to commit murder by his ambition on the one hand and his wife's goading on the other. This sets the stage for the rest of the play, and makes Macbeth's transformation into a bloody tyrant all the more powerful and dramatic.