Dean vs. Sam Winchester is coming to Supernatural, but that doesn't mean it'll be the final battle of the series. Supernatural is currently in its final season, coming to an end after 15 years of slaying paranormal nasties and stopping the apocalypse, and the stage is set for an epic, violent climax. Exactly who will be fighting for which side, however, remains to be seen. At the outset of Supernatural season 15, it appeared that God would be the ultimate villain of the series after he was revealed as the puppet master of the Winchesters' entire lives. But while God has certainly been a menacing presence in Supernatural's last season, the wind has been blowing in a very different direction.
Supernatural season 15 has revealed God's true plan, and in true Chuck Shurley style, it's all based around writing the best story possible to while away an eternal existence. God's looking for a Cain and Abel rerun, pitting Sam and Dean Winchester against each other in a tragic brother vs. brother battle with the threat of apocalypse handing over them. This grim fate has been teased in a number of vision sequences in which either Sam or Dean have lost control, forcing the other brother to put them down. The Winchesters are aware of God's intentions, thanks to a surprise hint from Lilith, but that doesn't necessarily mean they can avoid it.
As Dean himself admitted in Supernatural's latest episode, God won't stop provoking the brothers until he gets his desired "Winchester Bowl." This kind of definitive shadowing falls under the Chekhov's Gun principle; there's no point teasing a Winchester vs. Winchester clash unless it's actually going to happen. The fact that Supernatural season 15 has dedicated so much time to teasing the Winchesters fighting each other makes the prospect inevitable, rather than just possible. From an in-story perspective, Dean may also be correct when he suggests that God won't stop prodding until he achieves his goal. With Sam and God linked by their matching bullet wounds, the brothers can surely only resist the Lord's influence for so long.
In one sense, it could be argued that one Winchester killing the other is the best way for Supernatural to end. After taking down so many mighty villains over the course of 15 seasons, the only way to top all the previous big bads in terms of raising the stakes is to turn one of the main duo into the enemy. Such a clash would immediately draw the audience to edge of their collective seats and would provide a definitive ending from which there would be no going back.
But while Winchester Bowl might be unstoppable at this point, it's unlikely to be how Supernatural ultimately bows out. The threat of God still remains, after all, and will have to be dealt with one way or other. Chuck has been retroactively blamed for every misfortune that the Winchesters have ever suffered and it's he who is destined to be the final villain. For this to happen, whichever Winchester turns bad will have to push through God's influence, and this triumph of brotherly bonds over the power of God himself would be an uplifting moment for the concluding episode. Moreover, an ending in which Sam kills Dean or vice versa would be a depressing route for Supernatural to take, leaving a taste far more bitter than sweet after viewers invested so much time in the series.
Reassuringly, Supernatural appears to be well aware of this. Season 15's "Atomic Monsters" saw God reveal his plan for the Winchesters to their biggest fan, Becky Rosen. Like many real-life fans would be, Becky was aghast at the story God had concocted, proclaiming that this wasn't the ending people wanted for Sam and Dean. This completely meta reference is a clear indication that even though Sam vs. Dean looks incredibly likely, Supernatural has no intention of ending on such a controversial note.
Supernatural season 15 continues December 5th on The CW.