this started out as a response to
writingpathways's meta
here, and then it got too long to be a comment. it's been a very long time since i felt i had this much to say about an episode of spn, so i figured i'd let myself go to it. warnings for incoherence.
i hate that sam apparently thinks his brother's weak, but i can kind of see where it comes from - sam doesn't think dean is physically strong enough to take on lilith (sam's juiced on demon blood, dean isn't), but also sam is willing to cross a line that dean isn't. sam is willing to use the demon blood as a weapon, consequences be, er, damned, because he can separate what the weapon is (demonic) vs what he is (still human). whereas for dean, drinking a demon's blood makes you a demon, period, full stop, so it's completely off the table as a resource. but sam will use every possible weapon he can get his hands on to kill lilith, which in his mind makes him a stronger hunter. i think he's a lot more machiavellian, a lot more "end justifies the means". and dean's not. and in sam's mind that makes dean weak.
(everyone's got a line beyond which they won't go. dean and sam's lines are in different places and stop them at different points and prevent them from doing different things. for sam to cross dean's line makes him (in dean's eyes) inhuman, and for dean to have that line makes him (in sam's eyes) weak. but they see the world and their place in it differently, and i think something like this was bound to happen eventually, because they're different people who can't totally accept each other's worldviews. i don't think sam's completely wrong when he says dean doesn't know him. but i would hazard a guess that sam hasn't always made it easy for dean to learn, and also that dean might not want or be able to.)
i also think dean thinks sam is a monster, or is becoming one, but only because of the demon blood. you could tell that it ripped him up to say that to sam, to even entertain the possibility that his baby brother was turning into the kind of thing he'd normally hunt down and kill. back in s2, when sam was worried that his psychic power meant he'd go darkside, dean was convinced it wouldn't, because dean would protect him. dean thought he could save him. and i think now, dean blames the demon blood and ruby for getting sam to drink it, and once sam's got it out of his system, he'll be fine. he'll be sam again. i don't think dean honestly thinks sam is intrinsically monstrous (he didn't in s2, and he had some cause), but he does think the demon blood has made sam so. and i think where he's concerned, sam has passed the point of no return. i mean, sam won't listen to him, sam's consorting with demons and lying to him, sam is drinking a demon's blood, and sam tried to kill him. by itself dean might forgive that last one (like he did in buabs, when possessed!sam shot him), but with everything else? there's just too much that sam's done. dean can't forgive him the blood.
however. i think yelling "if you walk out that door, don't even think about coming back" is unforgiveable from sam's pov. we all know what happened after john said it, and there's no reason this should turn out differently. (
writingpathways suggested that that dean said it out of fear - as john did, knowing that sam leaving meant he wouldn't be able to protect him any more - and i don't entirely disagree.)
and i do think sam might be partly worried he could be turning into a monster, but is really more afraid of what dean thinks. the hallucinations were all manifestations of different parts of sam - wee!sam was his conscience and his lifelong desire to be normal, mary was his machiavellian hunter impulse, dean was probably his deepest fear. he hallucinated dean telling him "you've always been a monster, you're nothing to me", and in samworld, that's terrifying. part of him is still the little kid who wants his big brother to be proud of him. so when dean actually calls him a monster - even tho it kills dean to do it (he can't protect sam any more, he can't save him) - sam lashes out partly because he's afraid that he was right, and (basically) dean doesn't love him any more, and never did.
blood-drinking sammy's come a long way from the boy who was worried his visions would turn him evil. but i think it's a control issue. sam freaked out in s2 over something he couldn't control, but the demon blood was his choice. and he wasn't afraid of it until he realized he was becoming addicted and losing control, giving it up to an outside force (like the yed in s2). it's not just that he's afraid it's turning him into a monster (because i don't think he totally believes that - he's using the weapon, he's not becoming the weapon), but more that he's afraid it's turning him into a powerless junkie. which are two different things. when he was eighteen (give or take), sam left his dad and his brother and everything he knew and loved in an attempt to control his own destiny - this is not a guy who likes having someone/something else in charge, or who's worried about the consequences when he takes the reins. well, he might be concerned about the consequences, but he sure as shit isn't going to let them stop him.
(maybe i shouldn't be referring to the demon blood as a weapon. but i don't know what else to call it. it's like supercharged supernatural steroids. it's really more of a resource or a power boost kind of thing, but sam's using it like a weapon, or to turn himself into more of one, and whatever you want to call it, there's still a difference in how he sees it vs how dean sees it. sam thinks he's ok because his intentions are good - he wants to kill lilith and save the world and avenge dean going to hell, and aren't they hunters? isn't this what they do? - but for dean they're still leading him to hell because the tools he's using to act on those intentions are of hellish origin. like i said, different philosophies and ways of thinking about what they do and how they accomplish their ultimate goals.)
someone who isn't me can probably say all this more elegantly and succintly, but it wouldn't be me if there wasn't babble. babble is my superpower, i'm not even kidding. i hope it makes sense.