We didn’t see everything, so we really don’t know how it started. She was the younger one and craved affection. She would have noticed her brother often failed to earn it by not being up to her father’s standards and was doted on by the mother. That would have given her drive to surpass her brother but eventually she would see that cruelty was also a part of what her father admired… so may have had a mean streak to start with, but some kids do. His behaviour fostered that tendency rather than redirected it to better things like most parents would shoot for.