Abigail accuses Elizabeth of witchcraft for two reasons. She wants to get John Proctor back in a relationship with her by removing Elizabeth from the scenario. (This means Elizabeth will be put to death. Abigail is okay with that.) Also, she wants to clear herself of blame and erase the public memory of the events that started the witchcraft debacle in Salem in the first place. She wants people to forget she was dancing in the woods and drinking blood.
Mary Warren accuses Proctor of forcing her to wrongfully confess and to expose Abigail's lies. She does this because she cannot stand up to Abigail in the court and, apparently, blaming Proctor for her behavior is the only and best idea she can come up with to clear herself.