Helen was a trauma bond for Carol. Carol cannot truly love.
EDIT: It's not a trauma bond. It's an unhealthy attachment style.
Carol is incapable to truly loving anyone. She has a lot of anger. Yes...i know...her wife died and I'm taking that into consideration.
The vacation that she and Helen had in the ice hotel spoke volumes. Helen was trying to get her to enjoy the moment... enjoy BEING WITH HER. But Carol was all sour and whiny and couldn't even connect with Helen in that moment.
Fast forward to The Joining. When she met with the others who were immune, she couldn't even bond with any of them on any level. Yes, she wanted them to see her point of view, but it was either her way or she was taking he ball and going home.
Think about when Helen and Carol were together. Who else did Carol have in her life? Any friends? What about her fandom? Any true, joyful connection to her fans at the book signing? Helen was her everything. She created a world around Helen. But she needed Helen more than it was actual love. I actually thought Helen was simply the publicist who Carol happened to sleep with.
I'm estimating, but i think it was around 3-4 months after her wife dies that Carol sleeps with Zosia. 3-4 months. It certainly wasn't even a year. This was after seeing other people and having been given an opportunity to actually have a supportive group who may provide comfort for her.
But no. Carol chases away developing non-sexual possible friendships with people like her and chooses a sexual relationship with a creepy "I/we" who eats human flesh, when needed.
Carol is like many people who seek connection with one person who they make their everything. Carol had a dysfunctional attachment to Helen. Helen was likely the only person who could put up with Carol.
Much like Zosia.