Lady Macbeth underestimates the power of conscience. She thinks that it is easy to divorce oneself from one's actions, simply by not thinking about them (she tells Macbeth that thinking on things would make him crazy). She was never a woman devoid of feeling or conscience, and that is what makes her interesting. Even though she seems to think she is stronger in will than her husband, she did not think she had the spirit for the killing that she needed, otherwise she would not have asked the spirits to 'unsex' her and fill her with venom. She thinks she is freer of the 'milk of human kindness' than her husband, and she may well be, but she is not entirely free of her humanity, which explains her strange comment about being unable to kill Duncan because he 'looked like her father' as he was sleeping.
Underestimating the residual effects of our actions is a common human weakness. We can't really stave off stress by refusing to think about it, any more than we can change a stupid or hurtful thing we've done by pretending it didn't happen. Sure, we might be able to delude ourselves during the daylight hours by an act of will, but we can't control our subconscious mind. Sleep, if and when it comes, will let the full cause of our unrest surface.
Were she actually as inhuman as she liked to present herself as, Lady Macbeth would feel no pricks of conscience, but she's not.
Lady Macbeth grabs her chance and acts as the water and the sun to cultivate the seeds the witches planted in Macbeth's mind. Shakespeare begins to show us the future of these actions when Lady Macbeth says, "Had he not so closely resembled my father, I would have done it" about Duncan's murder. We get our first glimpse of her as a human being with a conscience with this quote.
As time passes, her reaction and the memory of the deed works its magic, and she sinks into a deep remorseful depression. Her lack of ability to move beyond the murder is reiterated in her quote about how they have risked all to get where they are (the crown), but it is all for nought. She is not content, and this is largely due to the lack of support and companionship of her husband. He pulls away from her to become his maniacal self, and she is left alone to reflect. Ultimately, she begins to sleepwalk and then takes her own life to end the suffering.
I think Shakespeare is showing us that most people have a conscience and that only so much evil can be done without them eventually paying the price for it. What we see is that although Lady Macbeth was the one who pushed and pushed for Macbeth to seize the throne, she eventually realized that her hands were far from clean - that she was a large reason for the deaths of so many people. Eventually the guilt was just too much for her and she lost her mind over it all.
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