One thing "Supernatural" has been very effective at over the years has been end off season cliffhangers, the final moments of the season finales often leaving you screaming and demanding to know what's next.
IF YOU HADN'T GUESSED BY THE TITLE - SPOILERS AHEAD FOR SEASONS 9 & 10 OF "SUPERNATURAL"
The end of the ninth season was no different with Dean (Jensen Ackles) dying but the Mark of Cain resurrecting him as a demon. Executive producer Jeremy Carver tells TVLine that the season begins with Sam going on his own journey in attempt to find his missing brother.
In fact, Carver says "Sam is so invested in finding his brother that it leads him to do some questionable things that will make him and, certainly, the audience wonder which one of these guys is the true monster." Carver adds that Dean the demon isn't a straightforward case of Dean under the influence of a demonic possession either:
"Dean is not possessed by anything. It’s his own soul that’s been twisted into a demon soul, so this demon is not anyone other than Dean. So every thought, every action, that’s Dean, as opposed to some other demon possessing him.
So while he’s a demon, that causes some complication of thought in him in that, “This is really me doing it.” It also motivates a certain bravado. “Yeah, this is me doing and owning it!” There’s a real period for Dean of deciding how dark he’s going to be and what kind of demon he’s going to be, because he can’t blame it on anyone else but himself.
And then afterwards is when it will have the most effect on him because it was him. It was his actions that led him to becoming a demon. He pushed it with the Mark. He pushed Sammy away at the end of last season. He said, “I can do this by myself.” So there’s a certain consequence to all this that is almost more interesting to us than the actual demon part.
I don’t think demon Dean is the nicest fellow around the block. And that’s by design. He’s not intended to be a particularly nice guy and that’s one of the things he struggles with. He knows how he’s acting. Sometimes, he’ll fight against that in odd ways. That’s the real conflict that’s within him right now when he’s a demon. Because it is him. He’s not unaware of what he’s doing. And although his soul is twisted, he is not without a certain amount of self-examination."
Carver says Dean's switch to demonhood is rooted in the mythology of the show and that even when he inevitably changes back, it's not going to be a clean break:
"Dean being a demon here ties into a deeper mythology in the show, that being the Mark of Cain and Cain. That’s really one of the places we started to get the idea. The show has made a point of saying how rooted these brothers are in, frankly, the mythology of the world, and to find another anchor that was literally around at the beginning of time made us feel like… It wasn’t so much the spectacular effect or whatever of Dean being a demon. It’s more just rooting him in something more final, something that will not necessarily totally go away even when/if he’s converted back from demonism."