She doesn't care that Darla's not a vamp any more. Cordelia knows what she was, what she is, and what she's done to Angel. She's still a monster, a demon, even if she wears a human skin and soul now. She's not the kind of evil that Cordelia fights now, but the kind that she has known all of her life – even before she knew about vampires.
Even as she wraps her arms around her and protects her from Angel's wrath, part of her cringes inside. Her appearance is proof that Angel has lied to her, that Darla ultimately won.
She hates that she cares what happens to her, that Darla is now one of the "helpless" that she has sworn to help, and that she is carrying Angel's child in her belly, stealing one more bit of him away that Cordelia can't put back. She wants to hold her down and push her hand inside, to hear her scream - pleasure, pain, it's all the same with her eyes closed. She wants to rip away her disguise, and pin her to the ground beneath her dainty Manolo Blahnik shoes, and drive the heel through her heart like a stake.