She... pretty much uses Saya of an extension of her own self-hate at this point. She likes feeling inferior to her because, in her mind, that's the way it should be and any time it gets lorded over her, intentionally or otherwise, she feels vindicated in her own self-disgust. She admires Saya, although I don't think it is *precisely* right to say she respects her. She certainly doesn't trust her, except she has taken that promise to kill Barbet incredibly seriously. She wouldn't be able to handle him being around and... because she really kind of projects on Saya a lot, would get a vicarious sort of thrill out of seeing it. I don't know if it really came across at the time but, when Carla got into the game she had one been alive for a few months, and she hasn't even really begun to deal with all the lingering feelings over him.

There is also the layer of her that really really dislikes being so beholden to Saya, doubly so because both she and Blonde owe her for helping them into the city. It isn't something she can just ignore, so it nags in the back of her mind all the time. She wants to assert herself, but there isn't really any way she feels she can do so. Probably why she is so grumpy she didn't get to look the cold-hearted Saya in the eye while she still had her own empty heart to match. She doesn't want to always be the worm.