Frankly, this story might be worth turning into a series. It might be fun to explore how this mother/daughter duo might evolve.

Does the daughter continue to be terrified of her mother and what she does ? Does she end up learning, taking part and enjoying it ? Does she end up outing her mother to escape ? Does she end up a victim ?
As for the mother, It would be fun to see how maybe she explores different killing methods, or maybe how she turns into a serial killer and starts to develop and refine a modus operandi. Or maybe even she follows her own suggestion and starts keeping her victims for longer and longer.

If you want variation, it could be fun narrating a little bit from the perspective of the eventual investigators, or of the victims, as you do so well in other stories.

tldr : I enjoyed it and would love to read more.