When Cas breaks in, Dean's expecting Alistair. He's giving in, giving up, handing himself over; this is the moment he gives up on everything he ever believed in, and he picks up a knife. Cas is so bright it almost hurts Dean to look at him, and he doesn't know if it's because of his grace or because he's still a good person, holding onto things that Dean's about to give up.

If this is even Cas. Dean hasn't gotten up yet, hasn't started hurting people, twisting their bodies and souls into something dark and painful; that's how Cas finds him the first time, Dean with his hands dirty and so unworthy. This isn't real. He hasn't realized it's a dream yet, even if he's aware of a then and a now; Hell is always real, always bright and painful, and it sucks Dean into believing in it too easily.

So if this is happening, this can't be Cas. Alistair sent Cas in to mess with him like he did Sam.

Dean flinches and pulls away, but there's nowhere for him to go and he knows it. He's bleeding, but his whole body hurts.

"No, please," he begs quietly, squeezing his eyes shut. "I told you; I give up. No more tricks, Alistair."