I did look through the demon!dean tag and couldn't find this one.  It's a one-shot, with Dean/Sam, but they're not brothers.  Dean is a demon (possibly the prince of hell?) and he has captured Sam.   Dean is engaged to Meg (It could possibly be Ruby, but I'm pretty sure it was Meg), and the story is told from her point of view as she is watching them.  Dean has possession of the Colt.



He shoots Meg with the Colt, framing Sam for the murder, so all the demons will make sure Sam doesn't escape.



Thanks for any help you can give!

ETA: Found in comments.