Got this idea while watching the ending scenes of 2.20 "What Is and What Should Never Be." There was this moment, right after Dean killed the djinn, that really caught my attention. Anyway, hope you like! The title is from a line in e e cummings's "All in green my love went riding."
All the muscles in his arms burned, and something was screwed up in one of his shoulder blades. Didn't matter, though, because that thing was going after Sammy and just hell no. He didn't come back to this shitty world to watch his brother die. So he twisted and pulled at his nearly-severed ropes, concentrating on Sam and the djinn. He felt one of the remaining pieces snap, and he was so close now.
He was tired, god so very tired. His head was still thick with visions of that other world. And he'd wrenched that shoulder even worse. He thought, for a moment, that he just couldn't get free.
Then he saw the blue light, saw the glowing hand reaching towards Sam's face.
Before he even knew what was happening, he'd snapped the last fibers of that rope. He didn't think as he scooped up Sam's fallen knife and plunged it into the djinn's neck. He just did it. Rotated the knife, felt the tension give way as flesh and cartilage tore. He watched the body fall to the floor with sick satisfaction.
Sam didn't mention that particular part of the incident again. Why should he? Dean had saved Sam's ass hundreds of times before, just as Sam had saved Dean's. It was normal. What wasn't normal was the level of desperation he'd had. Because it hadn't been just adrenaline fueling him those crucial few minutes; it had also been sheer terror.
He'd seen the other world, where Mom was alive and Jess was engaged and Sam had a degree and things were stable. There was normalcy and happiness there. It hurt so bad to come back from that place where he could mow the grass and have his mother fix him a sandwich. Duty and Sammy were the only reasons he chose this world over that one, even if it was only in his head.
He was thinking about staying, seriously considering it, until Jess and that other Sam started talking about watching Sam live a full, happy life. And it hit him—only imaginary Sam would get that happy life. Somewhere outside of the hallucination was real Sammy, his baby brother, and he couldn't leave his brother alone, not when…not when they were unsure about Sam's future. That'd been the clincher. He couldn't abandon Sam.
That's why he was so truly frightened when that sonuvabitch djinn started reaching his hand towards Sammy's face. Because he'd seen, and he knew how good it was.
He knew he wouldn't be enough reason for Sam to come back.