A/N - there are references to events in s02e14 Born Under a Bad Sign and s05e22 Swan Song, referencing Sam's prior possessions, violence committed at those times, attempted non-con toward Jo (and Sam by extension) and vague spoilers for S09e17, Mother's Little Helper. You've been warned.
The blow came from out of nowhere.
As Sam started to turn away from the counter, fresh cup of coffee in hand, a fist connected with his jaw, sending Sam crashing back against the counter and his mug to shatter against the wall, splattering hot coffee across the kitchen.
"The fuck, Dean?" Sam demanded, holding his shirt, soaked in scalding liquid, away from his skin.
"You fucking drugged me!" Dean roared, his fist pulled back to strike again.
"Yeah, I did! Valerian and melatonin, nothing that would hurt you."
Sam lunged forward, easily blocking Dean's next punch, long fingers closing around his brother's wrist. Dean yanked his arm back but Sam didn't let go, his bare feet slipping on the wet tile floor. Both brothers came crashing down hard in a tangle of limbs, twisting, trying to catch themselves, Sam's right torso hitting the ground with a resounding smack.
"Motherfucker!" Sam gasped, shoving Dean aside to roll onto his back and press a hand to his side.
"You had no right!" Dean shouted, his finger pointed in Sam's face.
Sam lifted his head and glared at him. "Really? Did you actually just say that? You?"
"Oh, don't even try to make this about that!" Dean said angrily, pushing himself up onto his knees. "I had you possessed to save your life. You ... I don't know why you drugged me, but it's not like it was a matter of life and death!"
"Maybe it was, Dean!" Sam snapped, still laying on the floor, both arms across his middle. "You hadn't slept in two nights! You start hallucinating that you're back in Purgatory in a bunker full of weapons, and you don't think someone could get hurt? Killing yourself trying to get Abbadon isn't going to undo everything that's been done!"
"What's it matter to you?" Dean asked, shifting around, trying to get out of the puddle of coffee that covered most of the floor. His jaw tightened, becoming as hard and cold as his eyes. "Just lock yourself in your room and let me kill myself! I'd be out of your way then."
"Lock myself ... what?" Sam pushed himself up on one elbow, still holding his side with the other hand. "You think that's what I want? You think I want you dead?"
"Well, you wouldn't have saved me!" Dean shrugged. "That's what you said. Same circumstances, you wouldn't have done it. And you don't want to be brothers any more. Just partners. Well, there's your chance to get a new partner. I bet Jody Mills would make a good hunter."
He pushed back, starting to rise to his feet, but Sam tangled his own feet around Dean's ankles and tugged, toppling the older man to sprawl back on the floor.
"No!" Sam shook his head, struggling to sit up, turning to lean against the wall. "Don't you run away from me this time! You sit here and listen to me, Dean!"
"I heard you," Dean said, coldly non-chalant, half reclining on his elbows. "Loud and clear the first time."
"You might have heard me, but you didn't listen!" Sam hissed, bending over at the waist for a moment before levering back up to look at his brother. "I said I wouldn't have let you be possessed. I didn't say I wouldn't have saved you if I could."
"Well, if we're done here ... " Dean started to rise again, but Sam's arm shot out to stop him.
"No, dammit, you're going to listen to me!" Sam shouted. He dropped his head, took a deep breath, and then another before he raised his head and glowered at his brother. "I wouldn't let you be possessed, because I've been possessed. I was the one Meg was wearing when she killed Steve Wandell. It's been almost ten years but I still remember every second of it. I remember being covered in his blood. I remember holding my fingers on his neck until his pulse stopped."
"Sam, that wasn't you," Dean said softly, his face smoothed into Sam's big brother again.
"I'm not finished!" Sam growled, his hand clenching his side while the other slapped the stone wall. "It wasn't my decision, but it was me! It was my body that killed him! It was my body that shot you! It was my body she tried to rape Jo with! And Jo was the only thing I was able to stop! And then Lucifer ... " Sam shuddered. "He killed innocent people with my body! He killed Bobby and Cas, and he tried to kill you! And I was there for all of it! It was my hands, Dean, mine! I had to watch my hands beating your head in and I didn't think I was going to be able to stop him! II was screaming for him to stop the whole time! And then Ezekiel ... Gadreel ... he killed Kevin and I wasn't even aware of it at the time. I remember it now, though. The last thing he said to me, he was asking for my help, and I killed him! I promised him when we first found him with the tablet that we wouldn't hurt him, and I was the one who killed him!"
"Sam, that's not on you. Kevin's on me." Dean insisted.
"It's on me too!" Sam surged up onto his knees, off the wall, his free hand coming up to his chest. "If I was dead, Gadreel couldn't have used me to kill him! I wouldn't see his face every time I lay down to sleep! That's why I wouldn't let you be possessed, Dean! It's not because I want you dead! It's not because I don't care about you! It's because sometimes living is worse than being dead!"
"Sam." Dean bit his lips, looked away for a moment, shook his head. "I'm sorry. I don't know what else I can say or do to show how sorry I am that Kevin died. That I let an angel lie to me and possess you the way it happened. But I'm not sorry I saved you, and I never will be."
Sam's shoulder's sagged, and the hand on his chest reached blindly toward the wall again. "That's the problem, Dean. I'm not worth other people getting hurt. I'm not worth saving at the cost of anyone else."
"You are to me."
Silence descended for a moment, broken when Sam went still in the process of taking a deep breath, and blew it out shakily instead.
"You're hurt." Dean sat up and reached for the hand Sam still held at his side. "What happened?"
"The demon who was working for Abbadon threw me." Sam winced as Dean's fingers probed the area he had been holding. "I think she may have cracked a rib, and I'm pretty sure that the fall we just took broke it through."
"Bottom rib?" Dean asked, all clinical efficiency except for his suddenly pale face.
"Feels like it." Sam nodded, gasping as Dean touched a sensitive area again.
"Dad always said ... "
"Bottom rib fractures are the ones that kill you." Sam finished.
"Come on." Dean slid an arm around his brother's back, lifting gently. "Let's get some dry clothes and I'm taking you to the hospital."
"We're not done, Dean." Sam said once he had his feet under him.
"Yeah, I know."
"We gotta deal with ... " Sam broke off in a hiss as he tried to straighten. "Mark of Cain."
"I know, Sammy," Dean repeated. "We'll get there."
Sam didn't say they were brothers again. Dean didn't ask. But Dean helped Sam put on dry clothes, took him to be checked out, brought him home, and tucked him into bed.
And the next day, the air in the bunker didn't seem quite so heavy.