She's crazy. She's so crazy that she kills her sister - and almost kills Judith - and has to be "put down" to keep the baby safe.
Tyreese: So what do we do?
Carol: I could leave with her.
Carol: We can't sleep with her and Judith under the same roof.
Tyreese: You wouldn't make it. Not on your own.
Carol: She can't be around other people.
Tyreese: Maybe we could try to help her. Talk her back somehow.
Carol: This is how she is. It was already there. I didn't see it.
Tyreese: How could you?
Carol: I should have seen it.
Tyreese: So maybe we go. Me and Judith.
Carol: You won't make it either. She can't be around other people.
However, thinking that zombies might be people doesn't necessarily mean you're crazy - after all, Hershel and Milton, among others, have felt the same way.