Here are three quotes from Shakespeare's The Tragedy of Macbeth that characterize Lady Macbeth as cruel and heartless. They are all from acts 1 and 2, during which Lady Macbeth speaks many lines that reveal her ambitious and callous nature.
"Come, you spirits/That tend on mortal thoughts, unsex me here,/And fill me from the crown to the toe top-full/Of direst cruelty" (act 1, scene 5).
In these lines, Lady Macbeth actively summons help from supernatural spirits to fill her with the dark qualities she feels she needs to achieve her goals. Interestingly, in this quote, Lady Macbeth suggests that she herself is aware that she does not possess the characteristics necessary to live as evilly as she desires; the lines foreshadow her later madness, which is evidence of her inability to cope with her guilt.
"Look like th' innocent flower,/ But be the serpent under ’t" (Act 1, scene 5).
In this line, Lady Macbeth advises her husband to be dishonest. She not only embraces dishonesty in herself but she attempts to cultivate it in others, displaying a cold nature of unusual depth.
"Wouldst thou have that/Which thou esteem’st the ornament of life,/And live a coward in thine own esteem,/Letting 'I dare not' wait upon 'I would, '/Like the poor cat i' th' adage?" (act 1, scene 7)
Here, Lady Macbeth shames and mocks her husband for his hesitation in carrying out his plan to murder the king. Macbeth displays some semblance of integrity in these moments, feeling a pang of guilt at the thought of betraying the king who has just honored him, but Lady Macbeth minimizes this integrity and accuses him of cowardice.