I don't think there's a third party.
I'm not even sure there's a second party, actually.
If there is a second or third party, I believe them to be accessories after the fact.
I think EB is an abusive, hostile, violent person who couldn't control her rage and whose only sense of personal power came from controlling, terrifying, torturing, and hurting the vulnerable. I think drugs made her even more whacko.
However, the one thing that gives me pause about EB acting alone is the location of Zahra. I'm not sure where she may have left her.