It could be a defense mechanism for sure, but her coldness and ruthlessness was there before she lost Sean, she was that way when Sean was still with her. Now he seemed to be in her circle, and probably because she had romantic feelings towards him. Saul is the same way, she has romantic feelings towards him and so she lets him in behind some of her defenses, but not all the way... and that not all the way part was probably due to the loss of Sean and being alone. But, again, even before all that she was cold and hard and out for herself.
You can see it in just noting that so many from Dunbar were abandoning ship. You can see it in the fact that she set up a secret weapons stash and didn't tell anyone while they were being attacked. She didn't say follow me to the basement, I have weapons and we can defend ourselves, she let everyone head up to get to a fire escape and draw away some of the biters while she went alone to the basement where she had prepared previous to stand alone and make sure she could at least survive. In the tower with Michael and Burt and Angel we see times when they wanted to leave someone behind and just look out for themselves, but when it came right down to it none of them could actually do it, CJ did.
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