Individual clip for 4x21, "When the Levee Breaks."
Holy shit.

Sam's got to be hallucinating. Ruby's got him addicted but good. Dean really needs to kill her like yesterday. Of course that doesn't erase Sam's responsibility. Yowza. This show really does go there, doesn't it?
When the Levee Breaks
I loves those clips.
As to Sam, who can really blame him.
Lets look at this logically.
He has never (even under extreme pressure, even ones deliberately set to make him go evil) has given into acts.
He has since he was 6 months old already had demon blood in his system, and not just junior league blood, but from one of the major agents of hell.
By the time he has lost Dean he knows that while he is immune to Lilith's energy powers he is still extremely vulnerable to her forces and he knows that she directly sees him as her primary threat (at least at this stage).
All he has is his knife. He's dead. Logically there is now way he can survive.
And of course he is in a mindset where he is just fine with that.
Ruby comes along, and gives him the means to not only help others but to gain vengeance on Lilith. All he has to do is amp his abilities (I assume that a process that YED didn't need to do since basically his own essence while he was alive was powerful enough to not only impower Sam but multiple beings at the same time.
And I would say at the time the season started I don't think an addiction had set in (reason later).
But once the season starts, suddenly the situation he is in has gotten vastly more complex.
Before he was only focused on killing lilith (and saving those he could, in that order of importance). But suddenly he has his brother back. Now he has to worry about Dean (Again), and face up to the harsh reality that he didn't save him. Clearly if he would have done what he has done in the last few months when Dean was still in his deal he probably would have been strong enough to stop any one (or thing) from taking Dean. That has to kill him with guilt (above and beyond that he couldn't save him in the first place, but that he probably could have but didn't). Then we have the news that not only does he have that to deal with, but that the literally end of humanity is now at hand (when it was before just a couple of hundred demons lose, of which just Sam and Dean took out a good sized chunk).
We then see Dean's reaction to what Sam has done (to a degree) and Sam gives it up, with apparently no side effects, at this point.
Sam then sees the increasing threat with Samhein, not only from demons but from Angels as well. And in a last resorts uses his abilities. And again at this point we see no signs of addiction (he has stopped feeding, and this last for quite a bit of time).
And even by his next encounter with a being that he has to utterly HATE beyond measure, he doesn't go back.
And even with him admitting the thing he wants now more then anything (Killing Lilith) he doesn't go back.
Its only after a rather normal case that highlights that Sam and Dean are in fact pretty much destined to die young. This is something he can't face. And I don't think its his mortality that pushes him to this choice, but Dean's. Again the idea of Dean dying is so much stronger then him personally losing his brother.
And when he makes this choice to go back to Ruby, he hasn't had any real side effects (except for the ones he had going back to season one, headaches). And he clearly sees that his abilities are absolutely needed, which he shortly gets proof of. I mean he has had demon blood in him his whole life and yet has done great good his whole life.
So by the time he really feels the addiction it is too late for him to stop.
So I don't see how Sam could rationally not do what he has done.