How is Lady Macbeth evil?
Lady MacBeth is not evil at the beginning of the play, but changes to become so once she hears of the witches' prophecies and once she samples the "o'er vaulting ambition" that so corrupts her husband. She is an active agent for change and for regicide, because MacBeth needs his lover and wife to "push him over the edge" so to speak. Lady MacBeth is the agent which convinces MacBeth to act. Upon reading MacBeth's letter, she starts to change, becoming evil since she feels MacBeth is "too full of the milk of human kindness" (1.5.17). To combat this, she must "pour [her] spirits in [MacBeth's] ear" as one would pour poison. In her famous invocation to the darker powers she asks for the evil spirits to "unsex" her there, take her milk for "gall" and fill her full "of direst cruelty" (1.5.50). Lady MacBeth's evil is becoming more and more apparent and it really didn't take much to make her so. Her actions of stealing into Duncan's chambers to "dress up" the murder scene is further proof of her depravity.