So I'll explain so you know where I'm going. Maggie used to be a guardian angel and Ciel was her charge, but when Ash and Angela moved to kill him and his family she tried to stop them but was stalled by being attacked. When she finally got there, wounded and weak, she saw Ciel was still in danger so she opened the gate so that Sebastian could hear his cry. For that crime of allow a demon into the mortal plain she was striped of her title but not before she found Bane and made the deal with him so she could keep protecting people.