Final answer:
Macbeth's metaphor 'I am in blood' signifies his deep entanglement in his crimes and the irreversible nature of his actions. Macbeth means that he is so deeply involved in violence and bloodshed that it would be as difficult to turn back as it would be to move forward.
Step-by-step explanation:
In the excerpt from The Tragedy of Macbeth, Act 3, Scene 4, Macbeth's metaphor "I am in blood" means that he is deeply entrenched in his murderous actions and the consequences that come with them. The statement suggests that he has committed acts that cannot be undone, and he is now so involved in his treacherous path that turning back would be as difficult as going forward. It reflects both an acknowledgment of guilt and a recognition of the inescapability of his situation.