Pain.
Excruciating pain was the most prominent feeling Dean was experiencing in that moment.
Betrayal came to a close second, but the pain he was experiencing had to be one of the worst things he ever felt – and he was stuck in Hell.
Why didn't Castiel just come to him? Why didn't he ask for help?
Dean would have given him help, advice, hell; he would've turned Heaven and Earth if it meant he could help Cas. He would've given the Angel anything, because Castiel was special to him.
So he asked, because he had to know.
"Look me in the eye and tell me you're not working with Crowley."
Dean saw the truth in Cas' eyes, but if he was honest with himself, he knew for quite some time. Everything was screaming at him to realize what's going on, but he cared for Cas too much, and refused to believe.
People say ignorance is bliss, and Dean would have agreed with them right then and there. The truth hurt so much and so bad, that for a moment he almost felt like he couldn't breathe.
He knew. He just really didn't want to believe it.
"Son of a bitch."