What's fascinating, of course, is that what came back from the dead is 100% pure unadulterated Sam Winchester. He's always been ruthless, and he's always been willing to sacrifice other people's wellbeing for his own: this is the boy who at 18 cut his ties with his only family because they would interfere with the life he'd chosen for himself. Now he's more mature and less selfish, but the ruthlessness is still there.
One of the many wonderful things about the first season was seeing Sam move through the grief he starts with -- at Jessica's death -- by coming to care about his brother again. Dean gets Sam to look outside himself, first with the ordinary (for them) tasks of hunting -- saving people who don't mean anything to them, which is not something that comes easily to Sam at first -- and then by pushing him to reconsider the family and his place within it, and the question of whether he can become what he wants and still be a Winchester. Or at least, whether he can be what he wants and still be Dean's brother. And the season culminates in Sam choosing his family over his own revenge: listening to Dean and not shooting John.
But think of "Faith": Sam has been willing to trade other people's lives for Dean's for a long time now. Sam can be very focused when he wants to be, and right now, he wants to be focused.
Sam in Season 2 was a little harder to track, because it was the season of Dean, and Dean's grief -- but it was also the season of Sam's fear, in particular, his fear of himself. And I think he's right to be afraid of himself, although not because there's anything particularly demonic about him. He's right to be afraid of himself because when he forgets to be afraid of himself, he starts killing people. Madison, Jake, now all the hosts of all these demons. And Sam doesn't care, because he doesn't have time to think of himself -- he's too busy thinking about Dean, and how to save Dean. He's selfless in exactly the wrong way.
Characters who do exactly the wrong thing because they love too deeply or care too much are a major kink of mine, so I am exceedingly happy with Sam right now. Or rather, with Sam's character arc; I also want to shake him gently and point out that his Winchester genes are leading him straight into in a whole new set of stupid choices. He is exactly as self-sacrificing as Dean or John, he's just going about it in a somewhat different way.
I wonder, a little, whether Sam thinks that if he can prove to Dean that he isn't worth it, Dean will help him find a way out of the deal -- but I think that's too Dean-like, and not the way Sam thinks. Sam knows he wants to live, and doesn't care how many rules he breaks. And really, what are a few dead demons (and dead humans) along the road, so long as he gets where he wants to in the end?
Oh, Sam. You are so smart, and yet so dumb.