Both the daughter and the mother are victims that have been gravely failed by the system.
I can't explain how someone could do this to a child, but I am aware of the harm that long term abuse can do to a young person. When we hear about grooming gangs, we often find that the victims then groom others themselves, because their abusers tell them to, but also because they become conditioned to have no idea of what is normal or acceptable anymore.
I doubt this girl was "fucking evil". More likely she had completely dissociated from reality, and any sense of what is rational and responsible was long gone.