The Avengers are not done with the Infinity War yet. Marvel's greatest heroes were up against their most powerful opponent yet in Avengers: Infinity War, and he will return to fight again in next year's as-of-yet-untitled Avengers 4. So, naturally, fans are examining every tiny detail in Infinity War to try to see if they can figure out: does Thanos die in Avengers 4, and more importantly, how will it happen? There may be a subtle detail in the movie that offers a big clue about how the Avengers will take Thanos down.
Spoiler alert: Don't keep reading if you have not seen Avengers: Infinity War yet. After all of the end credits finish crawling and the sole post-credits scene ends in a call to a new hero, Avengers: Infinity War truly ends with a promise: that Thanos will return in next summer's Avengers 4. Although the big finale scene in Infinity War killed off half of the Avengers team, fans know that the heroes will somehow return and save the day (I mean, how else would Spider-Man 2 and Guardians of the Galaxy 3 happen?). The real mystery here is not whether the Avengers will manage to defeat the all-powerful Thanos, but how. There are already some theories floating around that Doctor Strange foresaw the road to victory and guided the team towards it, or that Captain America will sacrifice himself to defeat Thanos, but a new theory points to the Marvel universe's long-hidden Soul Stone as the key to Thanos' demise.
Going into Avengers: Infinity War, the mystery of where the Soul Stone could be hiding was the biggest fan question. Despite countless creative fan theories, the movie surprised everyone by revealing the Soul Stone was hidden on the distant planet of Vormir, guarded by none other than Captain America: The First Avenger villain Red Skull. In order to obtain the stone, Thanos was forced to kill Gamora, revealing that he did truly love her. Although the Soul Stone has been at the center of fan theory for years, Thanos barely used it at all in Infinity War — only really using its ability to identify souls to combat Doctor Strange's illusory body doubles, and possibly at the end of the movie to have a brief conversation with Gamora's soul in the astral plane.
The Soul Stone may not have really had a central role in Infinity War, but fans think that it could be the key to Thanos' ruin in Avengers 4. As Redditor yeindaflesh points out, a common thread in Infinity War is that sacrificing a human life for a perceived greater good is never acceptable. Directly converse to Thanos' conviction that destroying life is necessary to ultimately maintain it, we see most of the Avengers allow Thanos to gain universe-destroying power in order to save a loved one: Loki steps up to defend Thor, Gamora reveals the Soul Stone's location to save Nebula, and of course, Doctor Strange hands over the Time Stone to save Tony Stark.
The moment in the movie that most directly contrasts this theme is when Thanos kills Gamora in order to get the Soul Stone, which some fans think may be a clue. Thanos' ruthless sacrifice of the one person he loves for the Soul Stone blatantly flies in the face of the rest of the movie's mores — add to that the fact that it is the one Infinity Stone that we haven't yet seen unleashed to its full potential (it has the ability to capture and trap souls), and it starts to sound pretty convincing the the Soul Stone may be the very thing that takes Thanos down.
We will have to wait until next summer when Avengers 4 comes out to see for sure what happens.