Shakespeare's use of blood imagery is a representation of the overwhelming guilt that Macbeth and Lady Macbeth feel after the murders. Like the use of weather to enhance the drama of the presence of evil in the early part of the play. The blood is a constant reminder of the wrongful acts that have been committed. The actions are so vile that they imprint Macbeth and Lady Macbeth with a new identity. They are vicious murders who are now being consumed by their guilt and being dragged into madness.
Macbeth and Lady Macbeth experience total isolation from everything that makes us human. Their existence is dominated by the forces of darkness they have unleashed.
Evil, once unleashed tarnishes everything in nature, therefore, a greater act of good is needed to put order back into the physical world. This act of good comes in the form of Malcolm who claims the throne of Scotland and the death of both Macbeth and Lady Macbeth. Thereby closing the circle of evil they opened with the surrender of their lives and the temporary gain they received by placing their trust in evil forces.
Once again, Shakespeare challenges us to recognize that if one accepts the invitation of evil, the cost that is paid will be much higher than the reward.