lindenharp

Your observation makes perfect sense. She is a victim of sorts, but she's also acting out of free will. The Master hasn't brainwashed her -- just given her a stage to play out her revenge fantasies. And he enjoys watching her moral disintegration, just as much as he enjoys watching wholescale murder. Possibly more. It reinforces his opinions about the worthlessness of the Doctor's pet species.

Thanks for the comments.