Note: Please note that the universe this story is set in is based on the endverse (and I've specifically used a scene from 5x04: The End as a parallel), but it not specifically the same as the endverse. There are many similarities and you can read it as the endverse if you want, but I've taken the liberty to make my own changes – (i.e. there is no Lucifer or past!Dean)
Dean's become a stranger to him.
The man he used to know; the man he fought beside, died for – the man he had loved, once – is gone. Castiel doesn't know the man who has taken his place.
He's been with the hunter for years now. They've survived where so many others have died. They fought – and when they lost Sam, they fought harder; always fighting to survive in a world that was already dead. They survived, but something had broken inside Dean, then, something fragile and irreplaceable, and Castiel sees the damage leaking through. Sam had been Dean's anchor to the world, his link to humanity – without him Castiel doesn't think Dean even knows how to be human. Not anymore.
He watches Dean as he tosses a beer to a man who has been with their group almost since the beginning, watches as Dean tells him "good job" and then blows his brains out. Castiel doesn't flinch; he's become too numb for that.
"Croat," Dean explains to the shocked expression of his companions, his face neutral. They believe him, they have no reason not to, but Castiel doesn't need to be a mind-reader to know that Dean is lying; he reads it in his posture easily enough. He's anything but surprised – the man had screwed up once too many, and Dean had been itching to put a bullet in his head for weeks.
Dean catches his eye and gives him a dark look, daring him to judge him for his little impromptu execution. There's nothing left of Dean in those eyes, nothing left of the man who had sacrificed his soul, once, in exchange for Sam's life – a man who had loved his brother so much that he couldn't bear to live in a world without him. But Sam is gone now, for good this time, and there's no bringing him back. The angels had gone and the Devil wouldn't deal. And in the place of that once desperate man there is only an empty shell – a broken man lost in a broken world, floating with no anchor to tether him down.
Somewhere in that train of thought, Castiel can't help but wonder how long it will be until Dean decides to put a bullet in his own head as well and end the pain for good. Something darker asks him what if it's your head he decides to puts that bullet in?
Watching Dean turn into this stranger, this thing, he doesn't recognize, Castiel decides it doesn't really matter who gets the bullet in the end; he just wishes it would be sooner rather than later.