Betty's disclosure that Abigail drank blood is evidence that she is driven to eliminate Goody Proctor. Betty's revelations help to highlight Abigail's hidden agenda. Prior to this, it is believed that Abigail was one- dimensional. However, as she confers with Mercy Lewis, it becomes clear that Abigail has another aspect to her identity. Betty's revelation shows that Abigail covets Proctor for herself. She is driven to do anything to make him hers. This involves the drinking of blood in the forest to cast a spell on Goody Proctor.
Betty's revelation clearly suggest that Abigail wants to get rid of Goody Proctor. Her revelation makes clear that Abigail has nothing but contempt and resentment towards Goody Proctor for being dismissed as the servant for the Proctor household. She seeks revenge for her dismissal and wishes to usurp her in both the leadership capacity of the Proctor household and as Proctor's wife. Betty's revelation helps to illuminate this aspect to Abigail's characterization.