The original script for Avengers: Infinity War could have completely ruined Thor's story for Avengers: Endgame, as it omitted one key scene. Infinity War was the first part of the conclusion of the MCU's Infinity saga, and the heroes were facing very high stakes. As the original six Avengers had been around for nearly ten years at this point, both Infinity War and Endgame were key to wrapping up some of their stories, or at least giving them some huge character-defining arcs. The latter was the case with Thor, but his story wasn't the same in the original draft.
Infinity War began by killing both Thor's best friend Heimdall (Idris Elba) and his brother Loki (Tom Hiddleston), which set the thunder god up for an emotional ride throughout the two films. He was dead set on vengeance against Thanos, in an unhealthy and unhinged way, almost regressing to the hubris he faced in his very first movie. This contributed to his Endgame narrative, but there was one defining scene in Infinity War that was almost lost, and without it, it could have killed Thor's overarching story.
The original script for Infinity War didn't include the scene with Eitri on Nivadellir, which contributed a lot to Thor's Endgame story, as well as to Infinity War in general. Instead, the writers had a scene where Rocket and Thor team up against a huge serpent. This would have likely been the Midgard serpent Jörmungandr from Norse mythology, who is said to unleash Ragnarok when it releases its tail. The writers realized this scene came across as too adventure-like, without any real heart or serving any character purpose, and thus they replaced it with the scene on Nivadellir, which worked perfectly for Thor's Endgame story.
Given that Thor: Ragnarok changed a lot about the doomsday of Norse mythology, Jörmungandr's inclusion could have been a nice nod to Thor's roots. However, it would have come at the cost of his character arc. Eitri and Nivadellir represent the extremities of Thanos' tyranny, and it was the first real glimpse of how easily Thanos can, and will, wipe out an entire planet. Thanos' Endgame plan operates under the guise that wiping out half of the population will mean there would finally be enough resources to go around. This calls into question if Thanos is truly all evil, but Eitri's heartbreaking story proves that he is. Nivadellir's fate is made worse when Eitri cries at Thor, ''You were supposed to protect us.'' Thor's key role, in the MCU and Norse beliefs, is that he is the ultimate protector of the nine realms, and Nivadellir is essentially evidence that he didn't properly execute his duties, which contributed to Thor's Endgame breakdown and then the full-circle introspection teased in Thor: Love and Thunder. None of this would have come to fruition, though, had the writers kept the Jörmungandr scene instead.
Thor's guilt propels him into taking the full force of a star just to wield the powerful weapon Stormbreaker, the ultimate symbol of protection, and the action almost contributes to his penance for the tragedy of Nivadellir. Yet, when the time comes to end Thanos for good, he tries to revel in the moment, which ultimately leads to his failure. The guilt Thor suffered with in Avengers: Endgame began with what he witnessed on Nivadellir in Avengers: Infinity War, and from this, sprung his surprising and strong character arc.
