So I'm going to get to the comments to my last two entry as soon as I can (I appreciate them all, and am super excited to go through them again!). But there's something I noticed that Ruby said and I think this whole debate about Sam kind of hinges on it.
So because drinking blood is such a galling idea, it's quite easy to subscribe to the idea that it will turn you into a monster. But if you think about it (and read the SPN Wiki) all demon blood has the power to do is enhance or give raw power. The amount of power itself may twist you to some degree (and, at the risk of giving an inch here, I think that's part of what's happened to Sam, though, in his defense, he's only used his power to save people thus far; people seem to be forgetting that while Castiel let Dean get his ass kicked and nearly killed by Alstair, Sam showed up and saved him), but that's all that demon blood can confer.
And we know that to be true because of the way hell works. They don't turn you into a demon by giving you demon blood to drink; if only it were that simple. It's not a snap-your-fingers transformation; it's not a transformation at all, in fact (though I'm betting Dean wishes it were, it would make it a lot easier to explain away his actions in hell). They "turn" you into a demon by forcing you to do twisted things like put people on a rack and rip them to pieces 24 hours a day 7 days a week 365 days a year. Ruby said, Lucifer 'twisted' Lilith into what she is, that is how a demon is made. By taking the human soul and not replacing with something bad (a la Joss Whedon), but wringing all the goodness out of it, so that you are essentially what you always were, you're just the worst version of that.
Power, however, is just power, and I think you could even make an argument for the thing inside of Sam not actually being any more evil than a grace. I think that may have even been their point in making Anna's first act (moving the chest of drawers) the same as Sam's fist unintended act. What Sam will become if he keeps drinking the blood is very powerful but not inherently evil; the potential evil is simply Sam's human reaction to that power. And yes, potentially this could twist him but it's not what Dean's suggesting. That's the big point; there is no metamorphosis as it were, Sam is still Sam, Sam will always be Sam, he's just Sam with a lot of power. Sam's personality could make it the twisting agent, but it doesn't make Sam inherently a monster just by being what it is in the way that Dean is saying it does, otherwise, Sam's been a monster all his life.
"It's not something that you're doing, it's what you are." No, Dean, it's about who Sam is no matter what's inside him. I mean, look at Anna. She's gotten her grace back and she can still feel, still make independent calls, still do everything she was doing - she's the parallel to Sam (like Ruby's the parallel to Dean,
it's even in the color scheme), and she proves that what you are has very little to do with who you are and what you choose to do.
So far, all Sam's done with his powers has been for good (he saved an entire town, does no one remember this?). Even killing Alastair - Dean was beaten and bloody on the floor, I think you can understand Sam's being a liiiiittle bit pissed there. And even if it was prideful to assume Dean couldn't get the job done, thank God Sam was worried and didn't trust the angels and decided to come charging in. Thank God, someone was worried about Dean the person there and didn't just see him as a tool (or an it). Honestly, If Dean couldn't take on Alastair, what reason has Sam been given to believe that Dean could take Lilith? Dean kind of conveniently left out the part where he sold his soul to heaven, and even so, the angels seem to have this way of not coming through in the way you'd want - sure, Jimmy's family was safe but Jimmy himself got shot. And the only reason Jimmy's family was safe, again, wasn't the angels, it was Sam - he kept his wife alive. That doesn't put the odds in Dean's favor here any more than they are in Sam's but at least Sam's power is his own and if he's playing with fire, it's fire made by one demon that he could kill, and not an army of righteous angels that control him.
At this point, it's either Sam kills Lilith with Ruby/some demon's blood or we trust the angels to come through in the way we want and not say, "oh, by the way, we'll protect Sam but you're dying. Sorry! Enjoy eternal life in heaven!". What happened in that hotel room was not a loyalty battle on Sam's end and it is so easy to say that Sam chose Ruby over Dean but it is also completely false. If it were that simple, Sam would never have practically begged Dean to come with him. Sam's not stupid. Sam is terrified. Like he said in 4.20, he's afraid of it himself but he also sees no plausible alternative and I defy you to come up with one. Sam has power right now and he knows how to replenish it; Dean has no control over whether he gets any kind of power or assistance or if that's even in the cards at all. The seals are breaking, the angels are sitting on their asses somewhere, and the world will end if someone doesn't step up to the plate.
Sam asked Dean to come so that Dean could do what it is that Dean was begging for in Sex and Violence; be a part of a hunting team, only this time, Sam takes the lead and Dean follows. How many times has Sam gone along with one of Dean's crazy-ass plans despite the fact that he thinks it's insane (I think he actually did it just last week; he was ready to do it in Sex and Violence, too, but Dean hung up on him)? Honestly, how many times has Sam thought the character they were trusting was shady, said so, but gone along anyway because it's the best scheme they have (even if it's not the best scheme they have)? If this plan backfires, well, I guess the angels could still uphold their end of the bargain, but at least Dean would be there to protect Sam and vice versa. Because staying away while your brother goes off with a demon to prove some kind of point is NOT HELPFUL or pragmatic, it's called being a self-righteous ass. Saying Dean wasn't strong enough to kill Lilith - what evidence does Sam have that he is? Faith in the angels? That's the alternative? I'm sorry if it hurts Dean's feelings but Alastair swept the floor with him - any plan Dean concocts now is much crazier than Sam's idea.
It's backwards. Watch the scene again. What happened in that room is that it came down to faith in the angels miraculously delivering or faith in Sam who has delivered all year and knows he needs Deans help in taking this one last step and Dean chose the angels because that means Dean's not weak. That means Dean gets to be the big hero. Because that was the sticking point, Sam's saying Dean wasn't strong enough. It's not Sam trusting Ruby because by asking Dean to come, Sam proved he doesn't trust Ruby, not to be his hunting partner. Maybe to be a walking blood bank and a kind of compass, but that's about it. And Dean knows that, if he thought Sam was really completely gone, he'd never have agreed to go with him on any condition. Sam even told Dean that he wants them to be brothers again.
Castiel never said that Sam would become evil, just something Dean would feel compelled to kill. Well, right now, all that means is that Sam becomes very powerful, because that seems to be enough. I want to say they're both being equally as bad, but they're not the same person and while Sam is being naive, he is doing what he feels he must because no one else is stepping up to do it. Sure, there's revenge mixed up in it, but what heroes motives were ever entirely pure?