Yes, our beloved angel of Thursday is the big bad.

Having absorbed all the souls of Purgatory (a hellish dimension of monster souls), Castiel goes from a trench-coated ally to a terrifying, god-complex deity. His reveal in "The Man Who Knew Too Much" (Episode 22) is Shakespearean tragedy. Cass didn’t betray the Winchesters out of malice; he did it out of love—and arrogance. He believed he was the only one strong enough to stop the archangel Raphael from restarting the Apocalypse.

The reveal in Episode 7, "Family Matters," is a game-changer: Sam is walking around without his soul. It was left behind in the Cage. This isn't just a plot device; it's a philosophical exploration of identity. Soulless Sam is hyper-competent, logical, and utterly detached from morality. He watches a woman get torn apart by a vampire not with horror, but with tactical analysis.