Oh Estelle. I guess... the crux of it is, Carla sees/saw the potential to foster something less spineless in her, and on a whim decided to try. She didn't really intend to become invested in it, but she kind of has because she and Estelle have shared that experience of being in a cage. And Carla doesn't want Estelle's own stupidity to see it repeated.

Carla isn't quite sure what to do with the fact that she is a hideous excuse for a human being though. She knows she isn't going to improve herself, but she also doesn't know how to build Estelle up either, so she just... tells her not to be so pathetic and doesn't let her 'delusions' go unchallenged. I think being in Estelle's body and... learning more about what her day to day is like will... do something. I'm not sure what, but Carla will feel like she knows her better.