Lady Macbeth has read her husband's letter concerning his meeting with the three weird sisters and their predictions. One of the witches had prophesized that Macbeth would become the king. Lady Macbeth is pleased but she knows that Macbeth may be ambitious but he couldn't possibly carry out the murder of King Duncan which would probably be necessary to give him the crown. She knows that without her intervention Macbeth will never be a king. The murder plan has already formed in her head when she learns that King Duncan is visiting Inverness, their castle, tonight.
"The raven himself is hoarse, that croaks the fatal entrance of Duncan under my battlements," Lady Macbeth says. A 'raven' is an ill omen and symbolizes death. It is clear that Lady Macbeth has no reservations about the murder of Duncan from the very beginning. But she knows Macbeth won't be so easy to persuade.
Her hand in this matter will be inevitable. So, she calls for the spirits to 'unsex' her. Seemingly, murder and such devilish acts are gender-related and the females are considered incapable of such evil. Lady Macbeth is asking the spirits to take away her femininity which is associated with tender, loving qualities and fill her with the most terrible cruelty.
Lady Macbeth asks to be made insensitive so that regrets have no way of reaching her heart and conscience doesn't get in the way of her plot. She asks for her milk to be replaced by 'gall'. Milk is not associated with evil and is a newborn's source of life. Lady Macbeth obviously wants to have nothing to do with anything that's not evil. She also calls for the 'thick night' or darkness, which is linked with evil, to shroud everything with darkness so that no one witnesses what...