I think Brett Partridge is Rj. It has been said over and over again in the show that a psycopathic killer likes to be involved in a murder investigation he himself committed (ex: SJK). In every RJ killing BP has been there or thereabouts and he likes to explain to PJ his theories (hence the involvement). I think he likes to taunt PJ by misleading him to see if PJ can see through his tricks.