I'm having some difficulty trying to figure out who RJ is. Although this is hardly a real suggestion, what about Volker?

Jane has had brief encounters with him, not good friends, but contact. He is very into Lisbon and enjoys playing games with her so I would actually think it's not to far fetched thinking he would kill someone for pleasure. Although in the episode where you see the kid with the car playing and then a guy gets strangled to death, he doesn't do anything. ALTHOUGH from my personal point of view he urges to do something, perhaps taking a life.

I know it's not a very trustworthy theory, but could someone perhaps dive into this theory? He has been in the focus of the latest season extremely much, which RJ was early on in the previous seasons, maybe he has taken RJs place?

I'll read upon the possibilities of this, but perhaps someone has already excluded him? Would be fun to know ^^

Have fun on your investigations :)