"Can you really beat her?" is the most important question.
It's the Big Thing that matters. It's making things right, finally.
But it's also a small, selfish thing and he knows that. He allows it this once – for the first and for the last time.
See, there are just certain things he doesn't want to experience (he can't).
And if he might get the luxury of not suffering through those, he'll lap salvation like a stray thirsty dog, which he is, if you put things into perspective and Castiel does put it alright – he knows his worth, he knows his place: there is none.
His face is getting ripped into pieces, were there a tomorrow, he would be painted with colors just like spring is radiant with flowers.
He is the shield, so he shields Dean. Takes one blow after another, it fills him to the brim with understanding: he is not the hero.
Dean doesn't even spare him a glance.
Somehow, that hurts more than the Devil's fist.
Mostly because there is something that he has to let sink in, even though he denies, and denies, and denies: Dean's longing is steadily waning, his love depleting.
He's being replaced with the poison Dean cradled in his arm for so long.
She ran in his veins, she boiled in his heart, they were locked with one key, they were one.
Soon, Castiel knows, Dean will miss her more than he ever missed him. He'll touch her where Dean will never touch him.
A bond is being replaced with another and it's not long before he will feel the old one break once and for all.
And this is what he doesn't want to live through – because the worst way to die is when you die in someone's heart.
He's pissed, he's jealous, he's alone and he gets beaten into puppy chow, while waves of the Darkness are washing him out of Dean's chest, out of his head, so subtle and slow Dean doesn't even notice he's changing.
But Castiel can feel it. This is how he's really dying. This is his least favorite way to die, and he tested plenty.
Lucifer knows and he promises peace. Castiel has questions, it's about small favors, actually:
Can you take my senses so I won't feel him slip away?
Can you take my eyes, so I won't watch him come with her?
Can you take my ears so I won't have to listen to the silence of all the things he'll never say to me?
Can you take my hands, so I won't crave to touch him?
Can you take my mouth so it won't thirst after him?