Castiel was never fully certain when the idea first took hold in his mind - whether it was when he realized he cared for Dean, when he rebelled for him, when he saw how much pain the Winchester boys were forced to deal with due to their lives as Hunters - it could have been any moment, really, but he never thought he would act on it. Oh, he was sure that, in centuries to come, once his friends had left him and he was all alone, alienated from both humanity and his own family, that he would regret never finally doing what he knew he should. For there was no question about it, it was indeed the right thing to do. It was what they deserved. But Cas was far too selfish, he thought, too cowardly to do what he knew he must. And so, when the idea formed in his head, he considered it for the briefest of moments, before hastily pushing it aside, to be hidden behind his doubts and fears and other, more important, concerns. But the idea never truly faded, and he never really forgot.
It was not until they were headed to SucroCorp, geared up to take down Dick Roman and destroy the Leviathans, that he began to see it as less of an idea, and more of a possibility. Of a reality. And when Dean drove that blade of a bone through his neck, and the three of them were together dragged into Purgatory, Castiel understood that it was not what he should do; it was something that must be done. As he watched over Dean's sleeping body, guarding him from the monsters in this brief reprieve from the terrors they faced together, he made a promise - to himself, and, more importantly, to Dean. He had to do this one thing, the only thing he could. He had to make it right.
~o*o~
They sit together in the hotel room Sam has rented. Team Free Will, together again at last. Dean and Cas have escaped Purgatory, they have found Sam, the Leviathans have all but disappeared, and they are safe. Until the next monster comes, and they are forced to take up arms once more. To fight once more. In truth, Castiel thinks, they are the opposite of Team Free Will, for they have never truly had a choice in the matter. It is always the three of them, they must fight, they must save the world, and they must shoulder the blame when they fail. Because they are among the few who know, who actually stand a chance against everything that's out there. But the Winchesters, they don't deserve this. They don't deserve this life of hardship, of loss, of guilt. They deserve better.
They are safe, Castiel thinks, and there will be no better time than now. With this conclusion, he rises, walking to Sam and placing a gentle hand on his forehead. He feels the cool trickle of memory leave the mind of the young man - but still just a boy - before him, as he collapses onto the bed, unconscious. Dean barely has time to begin a startled exclamation before he, too is falling backwards, leaving his memory behind.
He gazes down at the two sleeping figures, at his only friends in the world, and he realizes he cannot simply watch over them as they live their lives. For as long as he has known that the two of them deserve to be happy, to make their own choices, to live, he has never once entertained the thought that maybe, just maybe, he deserves that, too.
And so he reaches inside himself, past his vessel, into the deepest corner of his true being. He feels it there, glowing, pulsating slowly in his grasp. He thinks of all he's done; for his father, for his brothers and sisters, for humanity, for the two impossible boys that lay before him, and he pulls. And as every strand of himself connected to his Grace snaps, he feels a sharp spasm of pain. But he smiles past the agony, knowing that finally, finally, he is getting what he deserves: to be happy, to be loved, to be human.
