Dean was confused by what he meant by master key. He made a mental note to bitch Sam out for forgetting to tell him something so important. I mean if Lucifer say's Sam is to blame then it is most likely true. If Dean knew anything about the fallen archangel it was that he didn't outright lie. He would omit certain things, leave things up to your own imagination, but he wouldn't flat out voice a falsehood.

He also had a point about his gaining access to Cas. If Sam had waited like he'd asked then it might not of happened. Of course it wasn't like Dean had any control of Amara showing up and whisking off like that.

His eyes trailed over the figure descending the stairs because like it or not he did have a thing for Cas and while the one wearing him wasn't his best friend, visually...it was.

"I don't like it. You should leave." Dean said actually proud his voice came out stronger than he felt. He was at a supreme disadvantage and while he was slowly backing up he knew in the blink of an eye, Lucifer could be right in front of him. He tried to avoid jerky movements for now, best to try and keep him calm.