When I saw this bit in the promo, I thought it was Sam being shocked that Dean is now a demon. And that is part of it, but it’s mostly Sam being devastated that Dean is really dead and his body is being possessed by a random demon because at that moment that is what he believes. That little gasp he makes and the way his eyes are almost teary is heart breaking. And then he instantly gets angry and is ready to kill Crowley for doing that to his brother.