To Cain, Gina was nothing more than a machine.
It's the not knowing, that leads to the not caring.
What Gina meant to Cain, was her death. That one time she lowered her defenses to let someone into her mind, killed her. The one thing she was always against.
Cain herself was almost a Cylon, a perfectly oiled machine, driving towards it's goal, never sitting down, always running. That was her absolute strength, to lower the flaws of humanity, to keep them down and act with the natural instinct for survival, survival of the fittest, survival of the strongest gene.
It's the supreme irony that Cain was killed by the imperfection of humanity. Though, it shows that she NEVER in her life, ever had anything to do with emotions. This is why she just could not handle the love for Gina as a person, or the hatred for Gina as a machine.
She knew what was being done to Gina, she wanted it, for her own mind. It showed hatred and evil, and made her realize how "evil" Gina was. She couldn't be soft, once again, that would be disproving the survival of the fittest.
I believe that Cain joined the Fleet as a rough tough fighter bitch, never showed any emotions, and that lead her up to her Admiral status. The one time she showed emotions and love, got her killed.