Best. Show. On. Television. PERIOD.
It says a lot for the quality and twisted nature of a show when any main character of either gender could feasibly and believably turn out to be a serial rapist and murderer. And nobody has any clue who it'll turn out to be.
I still think it'll be some random person outside the main cast. At least that's the most logical solution. Besides the ether-adict who chopped up that bad lipo victim, have any other doctors had any vendettas against McNamara/Troy?