>>3493470>I had no luck in finding any fanfic with this scenario.
A brief look through AO3 shows me that most demon!Dean fics are A/B/O and have nothing to do with the canon Mark of Cain Deanmon. It didn't quite scratch the itch I was looking for, but Kintsukuroi by snarklyboojum was enjoyable and I would recommend. The problem with fanfiction is that the creme does not rise to the top. Searching by kudos and hits can help narrow the field, but a fic is just as likely to have a lot of kudos for being long as it is for being passable. Destiel is the ship with the single most fanfiction ever written for it, and it can be a chore to find the good stuff.
The thought of Deanmon tearing into Cas is delicious, though. Letting him know what a failure he is, how sick he is of cleaning up his messes. How he wishes Cas had just stayed in purgatory and done them all a favor. If he had the angels would never have fallen, and then everyone would have been better off. Deanmon telling Cas that he's known about his feelings from the start, and that he found it disgusting. That was the old Dean, though. Really, putting up with all his pining is pathetic, and it just makes him feel embarrassed for the angel. Maybe he'd indulge him just this once, if it would help Cas get over him and give up on this pathetic pipe dream of saving him. It would be done Dean's way, of course.