If they had a "demonic Dean" in the show at all next season, that would totally contradict that Ruby said it takes a couple hundred years before you lose your humanity.
Right ! Dean dies but comes back "slightly " demonic. Perhaps he's put under a post-hypnotic spell by Lilith to kill Sam the next time he sees him? So he runs off with Ruby to avoid that happening. He & Ruby continue killing demons while they hunt Lilith to get the spell removed. Dean fights to keep his humanity, he begins to find it easier to give in to his dark impulses, he's almost as much of a danger to the people he rescues as the demons he& Ruby hunts. Sam gets the Colt & needs it to kill Lilith . If he kills Lilith, Sam becomes King of Hell & can undo Dean's bargain. So the next season becomes Sam chasing after Dean & Ruby while everyone is chasing after Lilith with Dean staying one step ahead of Sam until the season finale. Sound good ?