The thing is - he should have seen this coming.
He saw these parts of Dean, long before anyone else did. The parts that Dean tried to bury for so long, the things that were added (and taken away, leaving empty holes where they'd been) in Hell; and the dark, gaping places that had been there long before Dean's forty years of torment.
He knew, while his grace was still intact and he still had the option before him to choose Heaven over Dean Winchester, that there was something cold and hard and vindictive hidden deep beneath the flirtatious smiles and the lighthearted wit.
He'd had fair warning, hadn't he?
And even as his powers faded right alongside every trace of softness and mercy the Righteous Man had once possessed, Castiel had stayed. Faithful, unwavering at Dean's side, even when it hurt so bad he could hardly stand it, and then when it hurt so bad he couldn't stand it, and buried the pain under drugs and sex and every mortal distraction he could find.
And in the quiet nights between battles, the rare occasions when he found no other company in his bed... Dean came to him.
He was ruthless and brutal, pinning Castiel down with his hands and his body and his fierce, biting words. He drank in Castiel's hurt like honeyed wine, kissed his tears with smiling lips.
It was as if he sought to fill the gaping maw of darkness in his own chest with what remained of Castiel's light, his grace - and there was so little left at this point. But Cas never could shut him out, never could push him away, even when he should have.
Besides, he'd seen this coming, or should have.
Every whispered vicious word, every grasping, painful touch that sought to hurt and tear and take, masking it with the thin veneer of a mockery of affection...
Castiel had seen it all before, in the dark places of Dean's soul.
He'd seen it - and he'd followed him headlong, willingly, into the darkness.