It represents Lady Macbeth's malaise and guilt awakening and driving her to insanity. In this scene, she "sees" blood on her hands, a metaphor for her guilt, and it cannot be washed off- the sin cannot be undone. After this scene, Lady Macbeth's strength runs out and is overcome with guilt to the point that she commits suicide.
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298-302 - Grendel snatched his first victim ripped him apart, cut his body into shreds with his claws.
