I'm pretty sure Cain was relatively "normal" prior to the attack. Sure, she was a go-getter. She rose in the ranks early in her career. Assuming she rose on merit alone, potential nefarious reasons aside, she probably was an excellent Commander. Then, two things happened - the sneak attack and the realization that Gina was a Six. The former hardened her into battle mode. We could assume this is something she trained for. The Gina thing is what put her over the edge into sociopath territory. The fact that she was sleeping with an enemy, who had access to her ship at the highest levels, responsible for the death of billions, and without ever having even suspected a little...well, I think that was a serious ego-crusher for Cain which put her on a one-way trip to dementia. No doubt she fancied herself a good judge of character up until that point.
To quote Rush "Emotion Detector":
Right to the heart of the matter
Right to the beautiful part
Illusions are painfully shattered
Right where discovery starts
In the secret wells of emotion
Buried deep in our hearts