As someone who made a review of Volume 2 said, Casey is a character of the Joker type (I wish she could be a fraction as well done as the Joker, of course, but I'm happy enough that she can be classified in his type). She was remarkably bad to start with, and through what has happened to her along the first volume and the time between its end and Volume 2, she has become a sadist psycho-bitch. She truly enjoys using her power to cause death and destruction. As you say, there is not too much humans can do to ensure their survival. It depends, exclusively, on how Casey is feeling at a certain moment.

The question of what would Kelly think is a very good one. It's hard to say, seeing how Kelly has been evolving. Deep inside, she would probably not care, but most likely she would let appearances guide her reaction and she would chastize Casey for her attitude. In the end, it would be nothing more than a way to try to show that she is the only one in charge, and hence the only one that can decide when it's right to kill and when it's not.

I hope that you keen enjoying what's coming.

Cheers!