the tap water or “holy water”

i still dont get why the demon would react violently to the holy water to cast doubt shes possessed. my theory is that holy water doesn’t have an effect on a demon that powerful so unable to tell if its real the demon incase it was acted like it was burning. because if the demon truly didnt want damian to know she was really possessed then why would she bring up her mothers death? which regan couldnt possibly know