It didn’t seem to matter that Dean knew that the Sam-shaped being before him wasn’t really his brother - even though the initial it’s Sam reaction has worn off, and he’d realized it couldn’t possibly be, it was hard to disconnect the words coming at him from his brother, voice and face and stupid floppy hair all Sam. But this wasn’t Sam.
Still, Dean had to wonder - that morbid curiosity like watching a car accident happen, you don’t really want to see but you don’t want to look away, imagination will fill in the gaps you miss, weave stories and endings that could be much worse than what you’d really see - how much of this was true? How much of this had Lucifer just pulled from Sam, somehow, stolen from his brother’s head and spilled back out in front of Dean?
All of it, Dean decided, fury bleeding out where the words wounded him, and he took a step backwards, one hand scrubbing across his face, half-turning away, trying one last time to pull everything in, keep it locked away, keep it controlled. Controlled anger, anger he could feed into a purpose - that was useful. He had to let it out with a purpose in mind, or he was no better than a child throwing a tantrum, just a lot more dangerous.
“We don't want you. You were dead and you should have stayed that way” and he lost that control he’d been trying so hard to build, whirling back, expression twisted into something that probably made him look almost inhuman, and there wasn’t even a second of hesitation before he was lashing out with the knife.
Maybe later he would take his time, stretch this not-Sam out on a rack and cut him apart, peel his skin away until he stopped looking like Sam - until Dean could get to whatever was underneath his brother’s image, make it hurt like it’s words were hurting him. For now, though, it was completely lacking in his usual finesse, no skill or style, just a quick slash, meant to make it stop.