The lack of a response, and the smile - that freakishly calm smile - made Dean’s skin crawl, and he opened his mouth to speak - but then Lucifer was... not there. Not gone, but not there, either, presence lingering in this space, all of Hell still silent and cowed around him. He spun to find the Devil’s new location, and froze when he saw the familiar shape of his brother coming closer.
“Sammy?”
Sam shouldn’t have been here. Sam was never here, even when it looked like he was. It was just another way the dreams mimicked Hell so precisely that sometimes he wondered if he was back there, whenever he slept. Demons got a kick out of making him think his brother was there - in Hell with him, on the rack beside his or on the one he was meant to work on that day. Sometimes it had been Sam’s shape they used while they tore him apart. They liked to try to break him that way, but he’d learned how to know the difference quickly. Black eyes didn’t belong in Sam’s face, and demons were crap at hiding, in Hell.
This, though, this looked like Sam - eyes and all. It threw him off for a moment, gave the words being said a chance to slip in deep. Sam was always his weakness - someone else could say the same things to him, and their words would bounce right off, but coming from Sam they turned to knives, to missiles honed in on just the right spots inside to break him. Sam was the one person who could make him feel safest, but he was close enough that anything he had to say that could have been harmful was, because there’s being cut on and then there’s having your heart cut out, and Dean’s been through both and he knows the difference now.
So, it doesn’t register, at first, that this is a trick. Not until Look at what you've turned into and You're nowhere near the brother I watched die all those years ago and I don't even know how to talk to you anymore - until Look at you. Look what you've done. All this time, Dad was so worried about me being the one to turn into a monster... And in the end, it turned out to be you words like knives carving out his heart and putting it on display for him so he would watch as it tried uselessly to keep him alive.
By the time he managed to choke out a harsh “You’re not Sam,” it was already too late - even if this wasn’t Sam, what he was saying was true. All of it. He and Sam, they weren’t the same anymore. This was why, he realized - because Sam didn’t know him anymore, and for once it wasn’t because Sam had changed. He had, and he’d become the monster not-Sam had said he was, even before this powers thing had screwed with his head.
And it had to be hurting Sam, seeing him so different. Being there and caring about someone who had gone so wrong.
If Sam didn’t care, Sam wouldn’t hurt.
Dean’s smirk was back, but it was a little twisted, a little broken. His hand tightened around the knife in his hand. Maybe this looked like Sam but it wasn’t Sam, and he’d long since gotten past his aversion to cutting open something with his brother’s face. “Nice try.”