For quite some time now, I've held the theory that Sam would go darkside, forcing Dean to do whatever it takes to bring him back from the brink. I expected it to happen before Dean's expiration date came due at the end of the third season, but now we have evidence that it was delayed for a more dramatically satisfying moment.
Casstiel puts it all on the table for Dean: Sam, as the sole survivor of Azazel's Antichrist Gladiator competition, is in danger of fulfilling whatever Azazel had in mind for his protege. Apparently the angels don't know that plan, and it's got everyone in a whirlwind over what Sam might do. Combine that with Lilith's ongoing plan to open the 66 seals and unleash Lucifer, and there are plenty of issues all around.
So the idea, as expected, is that Dean is the one person in the whole world who can turn Sam off the road to perdition, and in the process, perhaps stop LilithÃ¢â¬â¢s little run at glory. And considering that Sam is hiding a lot from Dean at the moment, it could be an uphill battle. Add to that the possibility that Lilith's plan was in direct competition with Azazel's plan, thus explaining why Lilith wanted Sam out of the picture, and things get ugly. Sam could appear to be doing the right thing, while all the while carving out opportunity for his own plans.
The nice thing about all of this is that it's consistent with everything that came before it. Ruby told Sam that there was something they didn't know about Mary and her circumstances, and sure enough, that was an understatement. Not only was Mary a total hottie (and Dean's reaction to that was classic), but she cut the deal that led to the events in November 1983. And the path to that decision was laid with unexpected and powerful moments.
Early on, I had speculated that John Winchester's family had been hunters, and that Azazel had attacked the family because of some history there. Close, but no cigar. I never would have guessed that Mary came from a family of hunters, or that she was desperate to escape that world. It adds endless layers of tragedy to a story that is already bathing in it.
One might ask, of course, if the truth is even worse than Dean realizes. Was Dean always the catalyst for the death of Mary's parents, John's initial death, and Mary's deal with Azazel? If Dean hadnÃ¢â¬â¢t been there, would any of it happened? Casstiel hints at the possibility that the Winchesters' ordeals have all been designed to serve a higher purpose, so from that point of view, God might have foreseen the necessity of having a pair like Sam and Dean at this point in the struggle.
But if Dean was always the one who triggered the family tragedy, how is that going to weigh on him? The Brothers Winchester already carry guilt around like it's going out of style, and this is a major deal. And I can definitely see this playing into Dean's anger with God over how the war against the demons has been handled, which in turn provides more grist for the drama mill.
None of which is a negative by any means; this is the kind of material that consistently puts "Supernatural" at the top of my list. Not a moment of this episode was wasted, and some of the revelations left me floored. In short, it was everything I could ever ask of an episode of "Supernatural" and more, and it sets the bar even higher for the rest of the season.