I really like Michelle Forbes as an actress, and I think she did a great job with Cain. That being said, I've rarely been as happy to see a villain take a round to the head as I was to see Cain get killed. Whatever part of Cain was human checked out a long time ago.
As for defending her: There's lines you don't cross, even in war, no matter how shitty you and your ship are having it. Makes me feel bad for the other female members of the Pegasus's crew. I got the feeling that the jackals lining up to rape Gina would have been deterred by something as simple as their victim being fully human.