This is a Supernatural AU in which Destiel is a thing, but there are definite differences. This entire scene takes place in the impala, with a major flashback.

They're in the impala driving when Cas turns on the radio. Carry on Wayward Son. That's what is blaring over the speakers. It takes a moment before Cas realizes that Dean isn't singing with him like he usually does … instead he's got the steering wheel in a white-knuckled grip. Cas was about to ask what was wrong until he saw the tears streaming down his lover's face.

Dean felt his whole world spinning, falling away as he saw the body crumpled on the floor. With out realizing he had moved he found himself kneeling next to the still form, gently rolling the body over so that it was lying on its back, cradled in Dean's arms. He was so pale, so cold, that Dean could barely bring himself to press his hand on the battered man's chest. He felt as if his chest was being ripped to shreds (he would know how that felt) as he felt for some sign of a heartbeat beneath his palm. He nearly cried when he felt the thready rhythm. Dean gently shook the battered, still form of his brother.

"Sammy! Come on Sammy, open your eyes! Please, just open your eyes! Sammy!"

Slowly Sam's eyelids fluttered open to reveal wide eyes that rolled frantically around the room. Several emotions flashed across his face as he began to struggle weakly against the strong grip that held him.

"Sam, calm down. It's ok, I've got you now. You're gonna be ok, Sammy, I'm going to take care of you. I'm gonna fix this."

"D-Dean? I'm so s-s-sorry Dean, I tried t-to st-t-top it…"

"Shhh. don't talk, Sammy. It's ok, you've got nothing to be sorry for. Shhh Sammy."

"I let… let you d-d-down, Dean. I always t-t-tried to make… to make you p-p-proud. I'm so s-s-sorry…" Sam's voice trailed off as a spasm of pain gripped his body, squeezing the air from his lungs. When it finally abated, he was left shaking uncontrollably. His eyes rolled wildly and his pale face twisted into an expression of panic. "Dean… Don't…D-don't g-g-g… Please…Don't leave m-me alone."

Dean felt his heart breaking as his brother struggled to speak.

" Sammy—Look at me Sammy! I'm not going anywhere. Don't worry, I've got you; I've got you Sammy. I'm not gonna let you go."

He pulled his brother closer to him as tears rolled unashamedly down his face. There were no words he knew to describe the anger and horror he experienced as he looked at what was done to his baby brother. One of his legs lay at an awkward angle and his face and arms were covered in bruises and lacerations. Dean saw red when he noted the obvious signs of torture marring his skin. What scared Dean the most was the deep crimson stain spreading along Sam's abdomen.

Sam's breathing hitched and his body began to spasm uncontrollably in his brother's arms. Trying to deny what he knew was happening, Dean pulled Sam closer and rocked him back and forth.

" Just hang on Sammy, I'll call Cas, he'll fix you. Just keep your eyes open, please Sammy." The body in his arms relaxed suddenly and Dean, his hand still pressed to Sam's chest, felt the weak heart beat begin to fade even further. "Sammy—Come on! Don't do this to me Sam; Don't leave. Sammy… Please."

Sam's face had relaxed and his gaze, startlingly clear, settled on Dean. " I can feel it, Dean. It's finally done…It's so peaceful. Dean..I…I'm so sorry."

Dean moved his hand from Sam's chest to clasp his face. " Don't…" Dean had to stop, his voice choked with emotion. He tried again. "Don't be sorry, Sammy. It's ok, trust me, it's ok. I….Just…just know, Sammy, that you are always and completely… f-forg-given. I'll always be proud of you." While he spoke Sam's eyes had drifted closed and a carefree look had come over his face. Dean just stared for a moment, a small part of his brain noting that his brother hadn't looked that peaceful since they were young children.

He shook his brother, trying to rouse him and ignoring the voice in his head that was saying 'it's too late'.

"SAMMY! NO! SAMMY WAKE UP! CAS…CAS I NEED YOU NOW!" Dimly Dean was aware of the flutter that indicated Cas' arrival and saw him kneel down next to Sam, whom Dean still clutched to his chest, rocking him. His desperate gaze met Cas' and Dean saw the sorrow written in them. His world shattered.

A terrible revelation rolls over Cas and he flicks off the radio with a distraught look on his face. He meets Deans eyes, a heartfelt apology being conveyed silently while he reaches out and grips his love's shoulder trying to console, to comfort his friend and wishing he could take it all back, fix what was broken.
Because Dean couldn't bear to hear that song, the perfect description of who Sammy is… was: the wayward son. The one person in the world that Dean had watched walk away from him countless times, only to come running at the first hint Sammy needed him. The brother that Dean had cradled in his arms as he moved away from him, somewhere Dean couldn't follow. Dean couldn't bear that song because Sammy was the wayward son Dean had promised his father to protect; the one man, in the end, he couldn't save.