I figured it out. It's NOT Number 6 that kills Cain...it's Sharon. In the first part they show her on the sitting with the Dr. and Adama. The camera pans down and shows her wrists that are brused.
Number 6 would be too easy a guess. Sharon would have TWO good reasons one Helo and teh Chief, second what was done to her.
Now how does she get on the Pegasus?