Understandable, the child villainness was already 9 when she was taken in so her common sense would have been that of a commoner's than a noble's. She reacted to it as a commoner child would, throw a tantrum.
And no one was able to fix that attitude either teach her to become an adult, neither common sense of a commoner or noble since she was neglected.
And because of this her attitude stayed as that of a commoner child even when growing up as no one was there to teach her. No one could even teach her to act like a noble child, much less a noble adult (such as using influence and status to threaten etc. rather than throw tantrums).
Very justifiable that she became a villain through and through.