Spoilers ahead for the November 14 episode of Supernatural Season 15, called "Proverbs 17:3."
Supernatural isn't holding back in raising the stakes for its 15th and final season, and the show has rarely been as serialized as in the first batch of episodes. "Proverbs 17:3" started out like just another case of werewolves attacking innocent people, with the slight twist of the werewolves being brothers. When they solved the case a little bit too quickly, they learned that Lilith was really behind the whole thing, as she'd been pulled from The Empty by God for a mission, and she revealed God's devastating true plan for the Winchesters.
The good news is that Lilith wasn't on a mission to kill the boys, and God actually told her she wasn't allowed. Her mission was instead to get the Equalizer back, and she destroyed it after realizing it was hidden in the Impala. The bad news is that, if God has his way, both Winchesters won't survive to the very end of the series. Let's get into what God's plan is, and then whether or not it could work.
God's Plan For The Winchesters
As it turns out, Sam has actually had an idea of God's plan for a while, although he didn't know it. Those dreams Sam has been having in which either he kills Dean or Dean kills him weren't symptoms of PTSD. No, they were actually glimpses of the fates of some of the others sets of Winchester brothers whose endings God already wrote in other worlds. Despite the fact that our Winchesters are supposedly God's favorites, Lilith told Dean that God's plan for them is the same as all the other Winchesters: one brother kills the other.
If God has his way, one Winchester will kill the other by the end of the series, and Sam's dreams/visions are proof that God has been very successful in executing that plan in other worlds. This makes sense as an ending that would horrify Becky, and even God's reduced powers didn't stop him from pulling Lilith out of The Empty. If he could do that, and the Winchesters no longer have the Equalizer, the odds may not be in the brothers' favor. So...
Could God's Plan For The Winchesters Work?
On the one hand, under normal (by Supernatural standards) circumstances, of course neither Sam nor Dean would kill the other, especially since they know this is God's plan and presumably can take measures to prevent it. On the other hand, God has clearly pulled it off in many other realities, and there's no reason to think those sets of Winchesters weren't once as close as these Winchesters. If he could do it before, why can't he do it again?
After all, the deaths in the other realities weren't exactly straightforward Cain vs. Abel situations. One brother would kill the other because there was demonic possession in play, and they weren't both themselves. Could Sam or Dean kill the other if that was the only way to stop a possession? Or some kind of darker fate? Or could one of them need to die to prevent something awful to happen, and they need the other to do it, like Rowena needed Sam to kill her?
Of course, it's worth noting that the werewolf brothers in "Proverbs 17:3" were both dead by the end of the episode. The first killed his brother to stop him from killing people and eating their hearts, then killed himself because he had become a monster as well. Could the Winchesters die in some kind of murder-suicide?
It's not quite the Butch Cassidy and the Sundance Kid ending that Jared Padalecki once pitched, but it's something! It would feel kind of wrong to me if one brother had to live with the other dead. Besides, it's not like death is necessarily the end in the Supernatural world. Whether Sam and Dean survive the series remains to be seen; Jensen Ackles had a hard time coming to terms with the ending at first, and Misha Collins has said that Supernatural won't have a "conventional happily-ever-after ending."
That said, Jared Padalecki promised that Supernatural will end with "a version of peace," so whatever happens, fans may be satisfied with the Winchesters' fates. Only time will tell. There's still a lot of the final season left before the grand finale, so be sure to tune in to The CW on Thursdays at 8 p.m. ET (for now, at least) to catch new episodes of Supernatural.