what was Mrs.poole's fault ?what caused it?

Mrs. Poole is a nurse that looks over the mentally ill Bertha. Mrs. Poole's one fault was that "she kept a private bottle of Gin by her". Now and again she would sample her bottle. Some nights Mrs.Poole would drink a little too much and fall asleep on her watch. Bertha would steal her keys and roam around the house "doing any wild mischief that she that came into her head." On this particular time she lit the governess's bed on fire and ended up burning the whole house down.