Sam's struggled to find his identity, find a place his whole life. He's doing now what we all do, taking what he has – his strengths, his flaws – and using them to find meaning and purpose. And I want to be devil's advocate here (pun intended) for a moment.
Something happened to him – think of it as a tragedy or a disability – and Sam is trying to not only live with it, but trying to make the best of it. He says in Metamorphosis:
And I'm just trying to take this -- this curse and make something good out of it. Because I have to.
Does he "have to"? No. He could ignore it, pretend it wasn't part of him, but I think conventional wisdom would hold that denial is rarely a path to self acceptance or a happy life. He's finding his own agency, becoming proactive, rather than reactive.
Sam is using what he has within a moral framework. He's using it to kill demons, while minimizing the harm to the people they possess. As he challenges Dean – isn't that good? He's also using it to try and kill Lilith – the demon responsible for Dean's death and torture. Now you may argue that revenge is not a productive and creative life path – but remember Sam grew up with John's mission of revenge as a model.
In fact society at large would support Sam's doing. Isn't he following the path of every hero – possibly sacrificing himself for the greater good? And sometimes heroes have to do things the rest of use would shy from. Isn't Sam like Jack Bauer except taller?
More precisely Sam is every superhero who takes the terrible accident that gave them super powers and uses them for good. Sam knows that with great power comes great responsibility. Now if only he'd wear more spandex.
But we keep being told by Dean that he's on a "dark path", a "slippery slope", and I think we need to look at why.
I think much of this is Dean's fear is of his brother changing, not into something evil, but just changing. We've seen how much of Dean's identity is tied up in being Sam's protector, his big brother. If Sam no longer needs protecting, if Sam is the one saving Dean's ass – then how does this affect Dean's sense of self? A sense of self already very fragile after his experience in Hell.
And I think we have a clue how an attack on Self scares Dean, because back in Malleus Maleficarum, it's from the point where Ruby tells him that Hell strips away who you are that he starts to really fear it. It wasn't the thought of pain that he feared, but loss of Self.
Of the change is specifically related to something which is demonic in origin. Certainly dean has struggled and come a long way from his earlier acceptance of the world view he inherited from John - that supernatural things are always evil. But how hard would it still be for him to accept something of this in Sam?
Dean might add that he's got some back up on this darkside evaluation though – Castiel has said if Dean didn't stop Sam, he would.
See, what that means, Sam -- that means that God doesn't want you doing this.
So normally you might think – okay Heaven thinks it's evil therefore it must be. But Heaven and Castiel have been shown not to be particularly reliable in these areas. Particularly with the revelations in 4.16, who's to say that this message didn't come from those Angels allied with Lucifer? Remember it was the traitor Uriel who threatened Sam directly. Why would he care about Sam using his powers – unless it threatened his plans?
Finally, we (and Dean) are concerned because of the alliance that has been forged with Ruby. We don't trust her because she's a demon, but aside from that – what basis do we have for being concerned about Sam's alliance with her?
Sam and Ruby's alliance started when she offered him a way save Dean. All along the way she has seemingly acted in Sam's interest – she's saved his life and Dean's, worked to help them, fixed the Colt, given Sam a reason to go on after Dean's death. Sure she's a demon, but aside from that she's not done anything obviously evil. I think we see all this in Sam's relationship with her – her actions engender trust, but he continues to be wary of her, and with all they’ve shared you would hardly mistake his demeanor towards her in their encounters as affectionate. Isn't he just using her as a means to an end?
Ah but you splutter, she's getting off on the whole blood feeding thing! What about her smirk? Well I think that scene is ambiguous – is Ruby so self-satisfied because Sam is becoming an evil demon and this is part of her evil master plan BWHAHAHA! Or because that part of her that's still human loves the fact that Sam needs her?
Are we also disturbed because Sam is enjoying his power? That we're uncomfortable because there's a sense that he's reveling in being stronger than Dean – at a time when, admittedly Dean really is weak and vulnerable? There was no doubting his concern when he was by Dean's bed, or challenging Castiel. Are we uncomfortable because we also are having to adjust in our relationship to Sam – no longer the angst-ridden, unsure kid, but a man who's taking control of his destiny, of the hand his been dealt.
I think there is no doubt that in accessing his powers has tapped into his own dark side. He is more arrogant and also less concerned about examining the moral consequences of his actions – for himself if for no on else. I think the line that disturbed me the most in 4.16 was when he told Ruby he didn’t think dean was up to torturing Alistair, because he'd changed since he'd been back from Hell. Not because he said dean was weak, he is, or that he shared this with Ruby. But the fact that he didn't question whether Dean torturing someone, even a demon, was a good thing.
Of course the point is Dean did choose to take on the role of Alistair's torturer. So what does that say about Dean?
In a way, this is a bit like when Sam left for college. He felt he had to leave in order to find himself, and that lead to a breakdown in his relationship with Dean. It wasn't that anyone was good or evil, just that people, and relationships change. And it took effort on both their parts for them to become brothers again. As it will now.
I think we as audience are reacting very like dean right now, and maybe we all just need to have a little more faith in Sammy (sorry Sam). I still think he'll end up wearing the spandex.
ETA: A lot of the way Sam is being viewed atm reminds me of early S1 talk around the time of Scarecrow, when Sam was framed as being selfish bc he'd gone off to collge, bc he wanted to pursue Jess's killer. Both very reasonable and understandable desires, but because he was putting what he wanted first, above what Dean wanted, this was seen as bad.
No spoilers for unaired episdoes please.