That is what the Black Fairy wanted to do. That is not her motivation. Why does she want to do that? Maybe she wants power like Rumpel? If so that wasn’t explained properly. Does she care about her son and grandson? She isn’t good to them. She never has been from what we have seen.
We saw in her episode that she created the dark curse to protect Rumpel from being killed, but once Rumpel was safe still wanted her power and she used that power to hurt Rumpel and Gideon. She wants Rumpel safe, but she treats him terribly. It pretty much just seems like she is a bad person and that’s it.