AN: This is the first Supernatural fic I've published so I hope I did Okay with the characters. It takes place during the episode The End when Dean's seeing his future and it's from future!Cas's point of view. Just my slashly interpretation of why future!Cas does what he does.
It hurt so much to see Dean the way he'd been. Surreal as hell, sure, but there he was, the Dean I'd first known, the Dean I missed, the Dean I still dreamed about, the Dean I'd fallen in love with. He was there beside me once again. I'd resigned myself to the fact that I'd lost my Dean and yet suddenly he was here. But this was not my Dean. This was the Dean of my past, brought forward by that bastard Zachariah, hardly better than a figment.
However, that didn't stop the pain of seeing them side-by-side – the Dean I'd lost and the Dean I'd lost him to. I'd never admit it aloud in a thousand years, but that difference was the reason I had become the way I was. Sure, it wasn't exactly Dean's fault that heaven had lost the war (though I suppose that was debatable), but this sad excuse for a life I was now living? That was Dean's fault, even though he didn't know it. The girls and the drugs, they were not "living it up" or any of the other crap I had told past Dean, hell, that I had told myself. No, they was because I could not stand what I had happened to my Dean.
I did this to myself because my Dean had become a stranger, cold and brutal and willing to do things he never would have dreamed of doing five years ago. I could hardly look at him now without it breaking my heart all over again. And so I tried to forget. I tired to forget that when we'd lost the war, I'd lost the man I'd silently loved.
And I blamed myself for it. Of course I blamed myself. If I'd stopped Sam. If I'd somehow been able to save Dean from himself. But there was no point dwelling on the past.
Except that now the past was standing right in front of me, a living, breathing reminder of what I had lost. It was no wonder I was so broken, the difference between the two Deans was staggering.
"Cas, you Okay?"
I looked up at the Dean I'd lost, the Dean my past self would lose. There was such concern in his eyes, something I'd long since seen die in my Dean's eyes.
"I know Zachariah wants you to see the future of the world if you say "no," but look at your future, Dean; look at what you've become, what you will become." I looked away, hardly able to force out the words. "I know saying "yes" will more or less kill you and I'm not saying that's what you should do, but whatever you say, don't let yourself become him. He's not you, Dean." I could feel tears in my eyes and I hoped the low light hid them from this lost Dean.
He frowned, looked at me, and then looked at his future self who was too busy with his guns to spare a glance for those around him.
"I know, Cas. I don't know what I'm gunna do and don't know what I'm gunna become, but it sure as hell isn't gunna be him."