Supernatural Season 10 Spoilers: Dean And Sam Becoming the New Villains?
If you thought Sam and Dean were the heroes of Supernatural, season 10 of the series might just make you change your mind, as the intrepid brothers seem to be slowly morphing into the newest villains on the show. Faithful viewers of the show might not find this very surprising, as Dean died in the finale of the last season and transformed into a demon with soulless black eyes.

The premiere episode of the tenth season further explored the transformation of Dean. The episode, simply titled "Black", finds him indulging in some karaoke while his brother Sam has no qualms about torturing a demon to discover where Dean is.
As IGN further points out, the new Dean appears to be free of guilt and enjoys his lifestyle as a demon. IGN credits Jensen Ackles who plays Dean for exploring the fun side of the character and doing it well. IGN in its report also mentions that Dean as a demon exhibits a lack of humanity and seems to like violence. He also seems to be willing to let his brother Sam die and doesn't bother to rescue him from the clutches of Cole.
As for the rest of the season, International Business Times notes that in the second episode "Reichenbach" Crowley played by (Mark Sheppard) understands that "the Mark of Cain needs to be fed". More information about the Mark Of Cain is provided by show runner Jeremy Carver. Who states to Entertainment Weekly that the Mark will overcome Dean and make him a full demon if it is not sated. This is why Crowley sends the demons for Dean to kill in the premiere, because killing helps sate the Mark. IBT, also notes that in the third episode titled "Soul Survivor," Crowley will ally with Castiel, after he finds that Dean as a demon is hard to control.