The passage in question appears in Act II, scene ii, shortly after Macbeth has returned from killing Duncan, still carrying the daggers, now bloody, that he had used in murdering the King. Blood covers Macbeth's hands, as well, a "filthy witness" to his crime. Macbeth is completely shaken and distraught by what he has done. Despite Lady Macbeth's orders, Macbeth refuses to return the daggers to Duncan's bedchamber and smear the King's attendants with blood, as had been their plan. Lady Macbeth then takes the daggers to return them, leaving Macbeth alone to deal with his blood-stained hands, which she had told him to wash:
Will all great Neptune's ocean wash this blood
Clean from my hand? No; this my hand will rather
The multitudinous seas incarnadine,
Making the green one red.
Through this hyperbole, Shakespeare emphasizes Macbeth's tremendous guilt. Figuratively speaking, there is enough of Duncan's blood on just one of Macbeth's hands to turn the color of an entire ocean red. In addition to its connotations of guilt, however, can be found an even more subtle theme in the passage. The ocean is naturally green in color. The blood on Macbeth's hands would turn the ocean from green to red, thus destroying the natural order of the universe itself, just as Macbeth's murdering the rightful King has destroyed the natural order. The hyperbole, then, serves to emphasize both the depth of Macbeth's guilt and the depth of the horror and criminality of his deed. In a monarchy, his is the worst of all possible sins.