Small spoilers for The Raid and Somewhere Between Heaven and Hell. I thought Dean rolled over too easily after Sam confessed to taking hunts from the BMoL, so I rewrote it to reflect what I wanted to see.
This is a work of derivative fiction and no copyright infringement intended.
Deal with the Devil
Dean dropped the reference books on the long table in the bunker library. They landed with a resounding thump, slight wisps of dust forced out from between the ancient pages. Sam didn't startle but Dean's booming entrance did get his attention. He looked up from his computer, recognized the closed off look on his brother's face and prepared for battle.
"Were you gonna tell or just keep sneaking around behind my back?" While it might have been phrased as a question, it sounded like an accusation.
"Tell you what?" Sam asked cautiously, a firm believer that he should not admit to anything until he knew of what he was being accused.
"That you were taking orders from those British bastards, just like Mom."
"Oh, that," Sam responded slowly. He knew his brother would eventually figure out what he was doing but had hoped he would find an opportunity to plead his case before that happened. He wasn't prepared for this particular argument, yet. Unfortunately, because Dean had found out before he had talked with him, his brother was likely to be twice as difficult to persuade. The older Winchester was a firm believer that a lie of omission was still a lie, at least when Sam was the one omitting.
"Yes, that. How long you been taking their hunts? What did they offer you, huh, Sam? Impressive weapons, bonding time with Mom or something you couldn't find here?" Dean threw out his arms to indicate the library and beyond that the entire bunker; the gesture possibly to include himself.
Sam closed his computer, stood and looked his brother directly in the eye. He hadn't done anything wrong and he would not be intimidated into feeling guilty; even if he did feel a little guilty for not telling Dean sooner. "What they offered was a chance for this to all be over. We killed the Alpha Vampire, Dean. You have to admit that was a good thing, that it will make a difference."
Dean shook his head in apparent disbelief. "Yeah, Sam, you killed the Alpha with the Colt that Mom stole from a Prince of Hell on orders from the Brits. A hunt where they didn't give her enough information or the right information…and I hope that wasn't deliberate. A hunt where the bad intel got a hunter killed and almost got Cas killed. A hunt where she kept quiet about stealing the Colt rather than trade it for Cas' life."
Sam leaned forward and splayed his hands on the table, his hair falling into his eyes. "Hunts go bad, Dean. It happens. Maybe they didn't have all the information they needed before giving the hunt to Mom. Maybe Mom was wrong to hide having the Colt and then giving it to the British. The important thing is that in the end, it worked out. The Colt was where it was needed, when it was needed, to kill the Alpha. It saved my life and Mom's and everyone else that was left in the complex."
Sam walked around the table to stand beside his brother. "The British Men of Letters have killed more monsters in the few months they've been here than we've taken out in the last eleven years. I know we have no reason to trust them and if anyone should hold a grudge it should be me, but you can't deny that they're making a difference. Working with them is a means to an end, a way for it to be over. If all the monsters were gone, there wouldn't be a need for hunters. No one else would need to live like we have."
Dean took a step back, away from his brother. "Sam, would you listen to yourself! Let's say you, Mom and those British dicks did kill all the monsters. Would that include Garth and his weird family of vegetarian werewolves? Do you remember Lenore? Weren't you the one to tell me that if the monsters didn't kill humans, we didn't kill them? Now you're set on wiping them all out?
"Remember Eve? You think that if all of her Alpha's were wiped out she might find a way to crawl out of Purgatory again? What about demons and other Hell creatures? Even if there were no monsters, there would still be evil things in the dark that need hunted. Your idyllic monster free fantasy won't change that."
Sam reached forward, planted his hand on his brother's arm, tried to bridge the physical distance Dean put between them. "Okay, you have some valid points; but consider this, with me on the inside, I can make sure that Garth and others like him are protected.
"And get this straight, Dean, I am not working for the British Men of Letters, I'm working with them and only for now. Why can't you just trust that I know what I'm doing!" he howled, frustrated at Dean's deliberate short-sightedness.
"Experience, Sam! Experience!" Dean shouted back, equally frustrated. He shook of his brother's hand off, took a deep breath and continued, "You've always known what you wanted and were willing to do whatever you needed to get it and just as willing to count the cost later. Grandpa Samuel, Mom, you…seem to have a single-minded willingness to sacrifice what's right for what's expedient.
"I'm not picking at scabs, Sam, and I'm not trying to pick a fight, but when Mom was rationalizing why she was working with those tea drinking bastards, all I was hearing was you justifying why you were working with Ruby, and then why you were working with Samuel and then working with Rowena to remove the Mark. You always say the same thing: that I should trust you because you know what you're doing."
Dean dropped his head, already weary of the conversation. "Sam, you are the smartest man I know, one of the best hunters I have ever met and more than that, you are a truly good person who tries hard to do the right thing.
"Except when you don't."
Dean took a deep breath and continued before his brother could respond. "I'm not saying I'm always right 'cause Chuck knows I've made enough mistakes to damn me twice over; but I know in my gut that I'm not wrong now and you are making a mistake. Working with the British Men of Letters is the same as working for them. They will call the shots and use you toward their ends, not yours. It's not right Sam, it's not, and I won't be a part of it." Dean avoided his brother's eyes. "I can't explain it, but I know whatever it is they want, what they are doing and the way they are doing it, is wrong and will end badly."
Sam tried again, sincere in his desire to convince his obstinate older sibling. "It's not wrong Dean. What are they doing on a big scale that we aren't doing on a small one?"
Dean shrugged and shook his head resignedly. "What I told Mom goes for you Sam; you're an adult and can make your own decisions. You want to work for them or with them or whatever you want to call it, I won't try to stop you but I won't help you."
"Please reconsider, Dean," Sam pleaded. "We're better when we're together. The Brits want both of us. More importantly, I want you. I need my brother."
Dean turned his back to Sam. "You, me, we keep making the same damn mistake; choose different roads rather than travel together. You've been down this road of good intentions before and you know where it leads."
"Please," Sam softly begged.
Dean ran his hand down his face, an internal struggle evident by the way he stared at the far wall for several minutes. "I won't work for or with them Sammy, but I'll work with you. I'll watch your back and fight at your side."
Dean started forward but paused in the doorway and without turning said, "When I told you to pick a side, I thought you'd pick mine."