Since the season premiere last week there's been a lot of talk about the state of Dean. Is he a demon or isn't he? We were certainly all led to believe this was the case from the S9 finale, but this dialog in 10x01 between Crowley and Dean raised some questions...
Dean: You sent those demons...to kill me?
Crowley: To keep you sharp.
Crowley: If it wasn't for me throwing demon chum your way, what do you think would have happened? The Mark needs to be sated. Otherwise -
Dean: Otherwise, I turn into a demon. Yeah, yeah. I sort of got that six weeks ago.
Crowley: Just trying to help.
Dean: You lied.
Crowley: Who do you think you’re talking to here? Does the Tin Man have a sheet-metal willy? Of course I lied.
However, in a recent interview with Jeremy Carver, by EW, the question of Dean's demonic status is brought up.
I want to clarify something. Crowley said the Mark of Cain had to be sated or else Dean goes full Demon? Isn’t he already?
Crowley is saying, “I’m keeping you sharp by giving you demons to kill and that is sating the Mark because the Mark needs to be sated, and if you don’t sate the Mark, then the Mark eventually takes you over even more.” So [Dean's] on this knife’s edge of yes, the Mark has already turned Dean into a demon, so if you don’t sort of feed the beast, the Mark will potentially take you over even more.
So I guess this does clear up things a bit more. Dean's a demon, but if he doesn't keep killing to feed the Mark he'll become even more of a demon. Whatever that means. All these 9 years a demon was a demon...pretty much. Sure we had grunt demons and then the higher ups (Azazel, Lilith). Of course, with the Mark, Dean's not just any demon. So maybe if he doesn't kill he will finally become a Knight of Hell? You would think Crowley would want that, wouldn't you? He talked about Dean joining him and ruling in hell. He certainly has some kind of plan up his sleeve.
My other thought/hope is that if Dean's not a full-on demon yet, this might be the way that Sam will be able to save his brother. Once the Mark truly takes hold of him it might be too late.