Supernatural 13x20 Sneak Peek "Unfinished Business"

Yeah i have a theory that most people are not going to like because basically my theory is that Sam is going to go to the 'darkside' now many people are thinking that, there is no way cause its just too obvious but to u i say, what other way could the writers end this? The last battle, the ultimate battle is going to be brother vs. brother. The ultimate struggle. I know there has many indications this season that Dean may have an evil side to him as well, with his recent trip to hell and all that tourting but u know Sam is giving into the evil that was forced into him when he was a baby....