Castiel stood in front of Dean, scowling up at him in a way that Dean had never seen before. The angel had been angry with him before, but this was different.

"Cas..." Dean started quietly, unsure of what to say. Castiel's gaze was piercing. "What the hell happened to you?"

Castiel looked like he didn't know whether to laugh or glare. "What exactly do you mean?"

Dean looked incredulous. "Cas, you joined Lucifer. Why the hell would you do that?"

Castiel's scowl intensified a fraction, and Dean resisted the urge to flinch. "Why not?"

"'Why not'?" Dean repeated, confused. "What do you mean 'why not'?"

"Why not join my brother?" Castiel clarified icily.

"Oh, I don't know, maybe because he's the Devil? You know, Satan?" Dean said sarcastically.

Castiel took a threatening step toward, making Dean take a step back. "Maybe. But he's better than you."

Dean frowned, pausing in his inching away to stare at the fallen angel across from him. "What do you mean, he's better than me?"

Castiel resisted the very human urge to roll his eyes. "You don't get it, do you?"

"Don't get what?" Dean demanded, easily answering the angel's question.

Castiel grabbed Dean's shirt and forced him against the wall, holding him there and glaring up at him. "I gave up everything for you." He said, anger slipping into his tone. "My home, my family, my power. Everything. And what do you give me?" He leaned forward, pressing Dean harder into the wall. "Nothing."

"Come on Cas," Dean tried to calm the angel down, his words slightly strained, "let's just-"

Cas threw Dean against the adjacent wall, leaning down to grab Dean's shirt and force him against it. "I dragged you out of Hell, I stuck by your side, I helped you, I fought for you, I killed for you. And you gave me nothing in return. Not even an ounce of gratitude. Not one. You just pushed me away." He pulled Dean closer then pushed him back against the wall, Dean's head colliding with the bricks. "I never mattered to you. I was always an outsider. Just another soldier in your sorry little army. Useful until no longer needed. Expendable."

"No, Cas, it's not like that." Dean tried, attempting to ignore the searing pain in his body from being thrown around. "Come on, man. I'm-"

"You're what? Sorry?" Castiel glared and threw Dean back against the wall, standing up. "It doesn't matter. I've given enough, Dean. I've given all I had, and I have nothing more to give."

With a flutter of wings, Castiel disappeared, leaving Dean alone in the alley.

I got this idea while in the shower. My friend HalfBloodAlchemist on deviantART has gotten me into Casifer and I was wondering, what would make little puppy-dog Cas join Big Bad Luci? And Dean HAS been an ass these past few episodes.
And thus, this was born.

Story behind this is that Dean said something to Cas that crossed the line, and Cas went to seek out Lucifer.

So Lucifer lets Castiel join him. Cas is unsure at first, but we all know Luci could talk his way out of prison. And he eventually convinced Cas that Sam and Dean never actually cared about him. And since Cas was already mad and hurt about how Dean has been acting/treating him, it just escalates with what Lucifer says.

And...yeah.

Hope you enjoyed. Because if you did, it means I can still write well even when I'm exhausted. :D