Castiel had gotten accustomed to human vision. Before Metatron’s betrayal, it had simply been easier when he was on Earth more than he was in Heaven, and then as a human, he’d had no other choice. He’d forgotten just how much it hurt to look at Sam and see his soul.
He never said anything to Sam, of course. Sam would assume it was the demon blood, and apologize, and start avoiding Cas. It was nothing of the sort, and Castiel would much rather deal with the pain of seeing Sam’s soul than deal with never seeing Sam at all.
The demon blood had very little to do with the pain of seeing Sam’s soul. It was the scars. Castiel had been able to heal or erase the physical scars that a lifetime of hunting should have left on Sam. He’d rebuilt Sam’s body from atoms, and he’d done it without the scars. If Sam had noticed that Castiel had been erasing the physical scars since, he’d never said anything to him. Dean had asked him one time, and decided it was cool.
Cas couldn’t do anything about the scars on Sam’s soul, though. Only God could do that. Castiel had inflicted some of them himself, and he felt responsible for more. Sam had forgiven him, over and over again, long after Castiel had felt he had any right to ask for forgiveness. Sam’s forgiveness didn’t erase the marks on his soul, though. His soul, which by rights should be either black smoke after the demon blood he’d consumed or gleaming and perfect, was mutilated, distorted, horribly malformed. Lucifer was an expert. He’d known exactly how far he could push the torture without destroying the soul.
How Sam wasn’t in constant anguish, Castiel didn’t understand. Yes, it was painful seeing Sam’s soul – the sympathetic pain of seeing someone he loved in what should be agony. It was also inspirational. If Sam could bear up under the damage to his soul, then Castiel could push through the pain in his wings and his heart.
Sam was putting his soul through more pain, now, watching Dean walk away. Sam had told him to go, agreed when Dean said he should, but it was carving into his already screaming soul. Castiel couldn’t agree with the choice – Sam and Dean belonged together, at each other’s side, looking after each other. They just needed to figure out how to do that without hurting each other.
It wouldn’t be easy, but even now, so furious with Dean that he couldn’t think straight, Sam’s first thoughts were of Dean. Sam let Cas heal his head wounds, and then forced a smile. “Thanks for sticking around to do that. I know you’ve got a mess with the angel civil war, and if there’s anything I can do to help, just tell me. Between that and making sure Dean’s okay…”
“Sam, you need me here. Your healing wasn’t complete, and forcing Gadreel out ripped open some metaphorical stitches. I can fix it and finish the job, but not all at once, not with my stolen grace.”
“Am I dying?”
“Not quickly, and not if I can help it. Which I can.”
“What happens to you when the grace runs out?”
“I’m not entirely certain. I believe that if I give it up, I’ll go back to being human. If I burn it out, I’ll die.”
“You have to be careful, Cas. Conserve it. Make it last until you can give it up.”
“I know. If it weren’t for that, I could heal you fully tonight. I’m sorry.”
“But I will die if you don’t do anything?”
“Yes. Eventually, in a few months, perhaps as much as a year. I know you would have preferred death to Gadreel’s possession, but I believe I can do this without hurting myself more than necessary. Please allow me.”
“Yeah. Be careful, if you do end up pushing yourself too far just tell me and I won’t blame you at all for stopping, but I’ve done the slow death thing once already and I’d rather not go through it again if you can save me like you always do.”