Why does Abigail Williams drink blood?

In The Crucible, Abigail drinks blood as part of a charm meant to kill Elizabeth Proctor, because she wants Elizabeth out of the way and hopes to have John Proctor for herself.

Abigail had an affair with John Proctor while she worked in the Proctor household, and Elizabeth dismissed her when she discovered it. That makes Elizabeth both Abigail’s enemy and the person blocking Abigail’s fantasy that John still belongs to her. When Abigail is alone with Proctor in Act 1, she insists that his feelings for her remain alive: “You loved me, John Proctor, and whatever sin it is, you love me yet!” Proctor rejects her, but Abigail clings to the idea that Elizabeth is the obstacle.

The act of drinking blood is revealed when Betty wakes and accuses Abigail of having gone further than the girls’ story of dancing and conjuring. Abigail’s reaction shows how dangerous the truth is: she slaps Betty and threatens the other girls with violence if they admit anything beyond the safer version of events. The point is that Abigail’s behavior in the forest was tied to real malice toward Elizabeth, rather than harmless mischief.

Later, Proctor says openly in court that Abigail wants Elizabeth dead: “She thinks to dance with me on my wife's grave!” His accusation connects the forest ritual, Abigail’s lies, and Elizabeth’s arrest to a larger pattern of vengeance. Abigail’s private desire becomes a public catastrophe because Salem’s fear of witchcraft gives her a way to channel her jealousy into harming others.