Thor's Thor: Ragnarok, directors Joe and Anthony Russo continued the God of Thunder's journey in the 2018 movie that also brought Thanos into the fold, after introducing him to the MCU years before. Knowing that he's one of the few heroes to defeat the Mad Titan, Thor took it upon himself to make sure that he was equipped with the right weapon by the time he faced the villain. Unfortunately, while he got the perfect opportunity to take Thanos down, a momentarily lapse in judgment resulted in Infinity War's tragic ending.
After the death of half of his people, including Loki and Heimdall, Thor vows revenge against Thanos. This keeps him from ing either of his fellow Avengers in Wakanda or Titan as he opts for a side journey to Nidavellir for the six Infinity Stones straight to Thanos' chest. This gave fans a sense of victory over the villain, only for the purple alien to squeeze in a finger snap, and the next thing Thor knew, half of life in the universe was already turning to dust.
Infinity War's plot twist was seamlessly executed - until the very end, fans didn't know what's going to happen. When the snap did happen, it was so quick; even Thor was confused by it for a moment. That being said, the God of Thunder's catastrophic mistake is actually foreshadowed a few years earlier in Age of Ultron. Set off by Scarlet Witch's mind manipulations, Thor finds himself transported to Asgard where a party is supposedly happening. There, Heimdall confronts Thor and accuses him of leading them to hell. He furthers it by calling Thor a destroyer. Confused by the whole situation, Thor starts to emanate lightning around his body, accidentally hitting a bystander who then disappears into thin air - similarly to how people got dusted in Infinity War.
Oftentimes, Age of Ultron is criticized for shoehorning story elements that don't have anything to do with its own story. Instead, their sole purpose is to tease what's in the cards for future MCU movies. Fans are looking back at 2015's Joss Whedon movie with some fondness as it set up a lot of things that happened in Infinity War and its direct sequel, Captain America wielding the Mjölnir and Thanos deciding to personally go after the Infinity Stones.
That being said, Avengers: Age of Ultron foreshadowing Thor's failure to kill Thanos isn't something that was pre-planned. The Russos Brothers, alongside writers Christopher Markus and Stephen McFeely, officially boarded the project in 2015, but they wouldn't start cracking the story until they finished their work on Captain America: Civil War. From there, the filmmakers may have revisited previous MCU movies for inspiration with how they were going to make Avengers: Infinity War and Endgame, ultimately giving them an idea on how to execute the now iconic Decimation scene.
