Lady Macbeth seems totally devoted to evil. She calls upon the forces of evil to unsex her, taking away the very compassion that is usually associated with the female sex - a truly frightening thing for a Jacobean audience whose image of womanhood was one of compassion and meekness. There is a dreadful destructiveness in her words, a fervour and commitment that is truly frightening: Lady Macbeth: 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, lines 41-4) There is only one aim in life: to achieve the goal of kingship for Macbeth, with her at his side. In the pursuit of this aim it becomes necessary to put aside any semblance of weakness or tenderness. She relies solely on her strength of will, made greater by the forces of evil upon which she calls.
She becomes the essence of evil: cruel, heartless, free of the morality of mankind, taking events into her own hands to create her own reality. Yet it is this denial of reality that will finally be her downfall. She is awesome in her commitment, yet pathetic in her belief in herself and the powers of darkness. From her first appearance we are made aware of the enormity of her desire to succeed at all costs - in spite of her husband's apparent virtuous and compassionate nature Lady Macbeth: Come thick night, And pall thee in the dunnest smoke of Hell, That my keen knife see not the wound it makes, Nor heaven peep through the blanket of the dark To cry, 'Hold, hold!' (Act 1, scene 5, lines 51-5) Lady Macbeth seems inherently evil, but we should consider that she is trapped by the very same device that leads to her husband's downfall.
Her dark and evil thoughts are prompted by Macbeth's letter. If he had not met the Witches, her ambition might also have lain dormant.