Sister Aloysius is not a stranger to sin. Before devoting her life to the church, Sister Aloysius was a willing participant, an instigator, an expert of sorts. She had a certain knack for sin, and has since confessed all and has been forgiven, her sins forgotten.
There are some sins that she cannot forget, some that demand her attention only in the crisp cover of night. It's the darkest sins that haunt her, that plague her with sleepless nights and vivid dreams.
She does her best to spare Sister James of the ghosts of these sins that creep around like demons. They stain her peripheral vision like a fog. Sister Aloysius has to stop herself, has to pray the sins away because she worries that tendrils of it will coil around the innocence of Sister James and infect her. If she's harsh to the young nun, it's because she must be. If she gives off the air of being unfeeling, it's because showing the opposite would be to contaminate Sister James with the shadows of her sins.
Sister Aloysius cannot act upon these sins as she'd have done in a former life. The temptation makes her skin buzz and tingle with awareness of what she'd given up previous to taking her vows, but she resists. She must resist. For God and for Sister James, she must force the darkness out of her mind.
She will protect Sister James. She will ensure that no harm befalls her. Sister Aloysius is not allowed to love her, but will endlessly give what little she can.