To do good without God's permission or authority is akin to doing evil. While it is not in itself evil to do good, God is indifferent at best towards any good done that is not done in His name or under His authority. God has a monopoly on recognizable good. All good done without Priesthood is competition against God.
This is a tenet of most religion and it's called Priesthood. Government is also a type of religion as the police are the only ones authorized to do good (defend themselves and others) or to commission others to do good (act contingent upon their authority). The nuns see the girl as evil despite the girl using her power only to do good. But the good she does is not in the name of God or by His authority. Therefore, it is not recognized as good, but as trespass upon God's monopoly on good.
This is a tenet of most religion and it's called Priesthood. Government is also a type of religion as the police are the only ones authorized to do good (defend themselves and others) or to commission others to do good (act contingent upon their authority). The nuns see the girl as evil despite the girl using her power only to do good. But the good she does is not in the name of God or by His authority. Therefore, it is not recognized as good, but as trespass upon God's monopoly on good.