I really don't think Castiel would do this, but it's the worst case scenario that's depressing the fuck out of me for no good reason, so here it is.
"You know, sometimes I wish you'd died."
Dean rests his head on his bent knees, back against the wall as he speaks into the faded denim of his clothes. His neck aches and he's so tired he can barely think. There's no such thing as rest anymore.
"I think it would have...I don't know...it would have proved something." His hands curl around his ankles. "I could have...It could have been over."
He feels salt water drip from his eyes into the fabric under his forehead.
"It's not about me being right." He sighs. "It's not about anything but..." he falters, knowing that there is something huge and intelligent and very angry with him somewhere in the air around him. "But then I could have kept you straight, in my head...you were always you, before." He says nonsensically, knowing what he means but unable to express it adequately. "No matter how sad you got, how shitty your deal was...you always wanted to do the right thing."
He tips his head back, a random spark of insanity setting his nerve endings and brain on fire.
"Was this the right thing!" he shouts, his voice cracking. "Was this in the plan! You fucking son of a..." A hot wave of air passes over him and he knows that he's not alone anymore, he's moved closer, becoming physical and solid just out of the range of Dean's eyes.
"If you were dead..." he whispers to the air in front of his mouth. Warm hands touch his own where they are still touching his ankles, burning palms touch his torso, cup his face – icy fingers slide under his clothes. Castiel has many hands, many faces that are all invisible. The shape of him that Dean feels without seeing, is not even vaguely human – just a greedy conglomeration of hands and mouths, all touching and feeling him. One such mouth kisses his cheek, his jaw, an edge of hunger in its feigned gentleness.
Somewhere, Castiel's consciousness, the being of thought and intent, is off destroying the world. It's a creative form of destruction, breaking and changing and resetting until some undreamt of perfection is achieved. If Castiel even knows what he's aiming for. This body, the one that prowls over Dean as he tries to sleep, rubbing against his naked skin or else, pulling at his clothes as he lies on the floor of his prison. This body, is just a creation that allows Castiel to be with him all the time, making sure Dean's protected, from himself, from his own traitorous thoughts, from ideas of escape. It's the body that takes from him the release Castiel still doesn't understand. Consummating the need the angel, turned deity, cannot unravel – so he chooses to placate it with a subconscious rendering of physical shape – endowed in all functionally relevant ways.
Dean wonders if Castiel, the mind part of him, the person part, even feels it when they fuck, down here in this little white room. Does he hear the words Dean whispers to him, the insults and pleas and promises and endearments? Does he care?
"If you were dead..." Dean whispers, shucking his clothes at the body's urging and lying prone while it rumbles happily, erotically, sliding over his skin in a rush of fur and silk and human skin, to grasp and enter and feel. "If you were dead I could think." Dean says, half lost already, to growing insanity and hopelessness.
"I'd know if I wanted this..."
The body presses down on him, and a blunt cry comes from one its mouths. Dean looks up, but there's nothing there, his own body lies supine beneath an invisible thing, lover or rapist he can't know for sure – he can't know what ideas, what sentiments Castiel has implanted within his mind. He can't know for sure.
"I'd know if I loved you..."
Somewhere very far away, Castiel's thoughts twitch, he feels his body somewhere else, coming undone. A vague feeling, like an ant crawling over his foot.