He stops fighting it when it starts to feel too good. He’s not who he was before, but he’s not a demon-army leader, either, so he guesses that this isn’t a happy medium but it’s close enough. Dean’s thought about killing him; he doesn’t know how he knows it, he just does, but Dean can’t do it because Sam’s his brother, his one big hang up still being save Sammy even if Sam knows he’s beyond saving. It’s not that he needs to be rescued; it’s not that he’s evil, because in the truer sense of the word he simply isn’t. He’s still a hunter; he still kills demons, just doesn’t feel such a large sense of remorse at the loss of the human vessel anymore.
Ruby was right, that time in the motel with the mirrors on the ceiling; there are casualties, there is collateral damage, and he hated it more than words could explain in the beginning. In the beginning, he sat in the bathroom some nights, door locked and sitting on the floor with his back against it, and held that gun; put it in his mouth more than once. He lost sleep, in the beginning, lost his appetite and too much weight, and started looking so much the worse for wear that Dean was honestly scared – not of him, never of him, but for him, for what was happening and what it might do to him; scared that it might kill Sam. In retrospect, it kind of almost did; a little longer with a little less sleep and food, he’d have probably been dragged to a hospital by Dean or Bobby, but it didn’t get that far, and not because of Dean or Bobby, but because of Ruby.
It was for Dean that Sam snapped out of it, but it was Ruby who sat down with the whiskey and pushed a glass half-empty at him and said “you can’t save Dean from Hell if you’re already dead”. He can make sacrifices if it means saving Dean, and that above everything else, is the reason he’s doing this at all.
He got used to it, even learned to enjoy it the way Dean used to enjoy killing evil things. He looked at it more like Ruby did; yeah, he killed a human by killing the demon in it, but in the end? He did that person a favor; they would have died slow and sticky, just like she said before, because you never really heal once that kind of evil has been in you for too long. He still remembers some of the moments, tiny little clicks of time, from when he was possessed, and they still ache and burn somewhere deep, and he’s glad they got it out of him before it had to chance to do more damage.
He’s still not sure why she’s helping because she’s never felt the need to justify her reasons to him, and he just learns to stop asking. Some things, maybe, are better left unknown. It’d be better if Dean didn’t know that Sam is fucking her now, because he doesn’t look at Sam the same; no excuse in Dean’s book for fucking demons, even if they’ve got a pretty body to be holed up in. He’s still fucking Dean, only he’s fucking her too, and he knows that Dean hates it, but he doesn’t try to stop Sam; he lets him do what he needs to. She sees things that Dean doesn’t because Sam won’t let him; she gets it when he’s ready to pick up the gun again and put it back in his mouth, and she makes him forget that all this is eating him up inside and reminds him to enjoy the fact they're winning this and that they’re saving Dean in the process. More often than not, she makes it feel good, makes him feel good.
She never complains when he fucks her too hard, when he puts her on her hands and knees and fucks her from behind, too hard and too fast and too deep, leaving bruises on her hips in the shapes of his fingers. Sometimes, when it’s been so bad that Sam sits for hours in complete silence, she’ll straddle him, kiss him slow and like it means something; she’ll undress him like she knows what it’s really like to be gentle, starting with the buttons on his shirt and pushing it from his shoulders when she’s done, ending with his belt and jeans, pushing them down his hips and off his legs. She rides him slow, hands on his chest, or pressing her breasts against him, kissing him deep. She whispers to him, so soft and almost soothing shh, shh, it’s okay or let go, Sam.
She tips her head back and closes her eyes, and lets him put his hands on her hips, move her faster if he needs it, and it almost hurts it’s so good. When he comes inside of her, explosive and sticky, she doesn’t complain, just rides it out with him.
When Dean fucks him, Sam begs, soft pleading noises and please, oh god, Dean, please. It’s a soft, slow-burn at first, when Dean’s inside of him, and then it melts to something else entirely, and he’s arching up against his brother, hands tightening on Dean’s arms, his shoulders, anywhere he can grab and hold on tightly to, and Dean never complains, either. With Dean, when Sam comes it’s so good it feels like he might stop breathing, and for a second it could be like it was, but it’s not, and they both know it. So Dean stays arms tight around Sam until the shaking subsides.
Dean he’s loved his whole life, for as long as he knew what it meant to love someone and even before that, it’s always been Dean, and Dean’s never had to say it for Sam to know that he feels the same way.
In some twisted way, he thinks she might love him; he doesn’t know how he knows that either, but he just does. What’s even more twisted is that he might even love her some sick way right back.