"Oh, he's still boring. A dull knife, knowing exactly where to cut."

Honestly he'd likely be easier to ignore, once they just turn off everything he says as useless. When you only spit vitriol Dean and nothing of substance, people stop listening. Word of Advice from Lucifer.

"He's not particularly new. It's not unheard of for a human to become corrupted on Earth and become a demon, it's just extremely unlikely these days. Typically they're made in Hell, and then they possess a human. They get exorcised, they just hop into another vessel; they don't need permission like angels do. Dean just is. His soul, if there's anything left to it, must be more tattered than it was when he was in Hell."

Pause.

"Oh yeah, he was in Hell, once. It was... mmmmm... kind of a long story, actually. Prophecy is like that." Locks and their requirements are like that. "But eventually he broke and took up torturing other souls so that he wouldn't be tortured anymore. Learned under the Grand Torturer himself." Why he knows exactly where to spin his knife and his words. "Excellent student, so I've heard." Wasted potential until now, apparently.

Lucifer divulges this information like he's discussing the weather. It's just normal to him.