"Don't you fuckin dare, Cas." Dean growled through the blood pouring out of his mouth. Cas held the dying man in his lap, Sam hovering over his shoulder.
"Dean. I can save you Dean. We'll figure something out." He cried as he felt the blood welling up under his fingers. The gunshot wound would kill him. Cas knew that for certain, but he had the power to save him. He could do it. Sam kneeled beside him and shook Dean.
"Let him save you." He shouted. Dean smiled and raised his hand to Sam's face.
"Sammy," he smiled, revealing teeth reddened with blood. "You have a good life, man. I love you." Dean turned his gaze to Cas and placed his bloodied hand on the angel's cheek. "Take care of him Cas. You hear? I swear to god I'll come back and haunt your ass if you don't." His laughter broke into hacking coughs, spewing blood on Sam's plaid shirt. "I love both of you."
Those were Dean Winchester's last words. Sam and Cas burned his body at midnight that night, just two hours after he'd taken a gunshot wound to the chest. God had warned Castiel that if he interfered with another Winchester death, then both brothers would be doomed to a fate not even Castiel knew of. They were unnatural beings, having died more than was acceptable. They'd run out of luck, and that was how it ended. A gunshot from a dark corner of a fake haunted house.
As the body burned on the pyre, Sam turned to him. Cas could see the anger and shock in Sam's eyes. Whatever happened next, it was not going to be good for him. Sam charged him, slamming him against a broad oak tree near the burning site. His eyes were alive with rage and streaming tears. "Why the fuck didn't you save him?" Sam charged as he pushed Castiel hard against the tree. The angel could have smited Sam where he stood, but he knew what he was feeling. If Cas were in his position, then he would be doing the exact same thing. He explained as calmly as he could, but the tears in his own eyes would make it difficult.
"He didn't wish to be saved. It would damn both of you. He wanted you to live. He is in heaven."
"Don't give me that bullshit about being in a better place. You could have saved him. Worked something out."
Cas paused for a few moments before answering. Sam would not like the answer. "I could have, but I didn't." Cas was only telling Sam what he wanted to here. He needed someone to blame, and Cas was only too happy to let himself be the scapegoat. It had nothing to do with the guilt he felt already. Nothing at all.
They didn't speak again after that, but Cas watched Sam like his brother had asked. He was there when Sam grieved, worked, married, had children, grandkids, and finally when the old hunter died. Cas stood over Sam's hospital bed beside his friends and family when he drew his final breaths. Old age had taken him. Cas smiled down at him. He'd accomplished a lot in his life, and he knew Dean was proud of his baby brother who had eclipsed him in age by decades.
He sent a message to Dean in his heaven, "You're baby brother's coming home." Cas waited for a reply, but he was greeted with the sounds of tears, questions, and laughter. A mix of elation and grief flooded his connection with Dean. Sam was already there.