We only learn that three of the characters killed someone, Grant, Anna, and Luke (although he claims his is in self-defense). Alex was the control subject (he never says anything about killing anyone only that he is divorced). When they are all talking there are flashes to pictures and a word or words, when they flash on Jordans word it is rational. My belief is that she was there to be the voice of reason; she also was the one smart enough to outlast everyone else. This in turn allowed her to outwit her captor.
In a deleted scene Jordon reveals to Grant that she was raped in her past. One day, her rapist ended up on her operating table and she made sure an "accident" happened so that he died.
