Why is she keeping her alive anyway?

This is not like in Misery, where the abductor actually has a reason to keep her captive alive (to finish the novel). Here, there doesn't seem to be a reason. She doesn't torture her for pleasure, like in those torture porn movies. She doesn't ask her to help her with anything. It is hinted that she may poison her slowly to death, like she did to her mother. But this is not like you have to keep someone alive to avoid suspicion. The suspicion is ALREADY present since other people know Elizabeth is missing. So why not just kill her right away? The longer she keeps her alive, the greater the chance her evil plot is found out, and that's exactly what happens.