Thor was unable to kill Thanos in Avengers: Infinity War, but it turns out that wasn’t his dumbest mistake. Infinity War was the Avengers’ first real loss in the Marvel Cinematic Universe. After Thor failed to kill Thanos, the Mad Titan successfully wiped out half of all life. While being unable to kill a god-like being is understandable, Thor made a different, objectively silly, mistake along the way.
Infinity War marked the first meetup in the MCU for Thor and the Guardians of the Galaxy. Following an attack from Thanos on the Asgardians, Thor is left floating through space until the Guardians eventually come along and pick him up. After finding out Thanos is going after all the Infinity Stones, Peter Quill, Drax, Gamora, and Mantis head to Knowhere to try to protect the Reality Stone. Thor takes Rocket and Groot to another location, Nidavellir, to get a Thanos-killing weapon, which turns out to be an enchanted ax called Stormbreaker.
When he first meets the Guardians, Thor tells them that two Infinity Stones are safe on Earth with the Avengers, Earth’s Mightiest Heroes. He doesn’t, however, go into detail about who the Avengers are or what they look like. This turns into a horrible miscommunication. Later on Titan, Thanos’ home planet, Quill, Drax, and Mantis get into a fight with Tony Stark, Peter Parker, and Doctor Strange. With Thor failing to mention pertinent Avengers details to the Guardians, each group mistakenly thinks the other works for Thanos. The battle nearly turns deadly until Strange finally figures out they’re all on the same side.
This fight didn’t have to happen. If Thor had simply shared some details—any details—about the Avengers, Iron Man, or Doctor Strange, all issues could’ve been avoided. Since the MCU doesn’t have an inexhaustible number of genius, billionaire, playboy, philanthropists, the Guardians would likely have realized who Iron Man was before starting the conflict. With Stark willing to do anything to protect Parker and Drax the Destroyer being, well, a destroyer, it was fortunate for everyone that things didn’t escalate further.
Even though Thor should’ve shared some details about the Avengers, a lot was out of his control. When meeting the Guardians, it’s clear Thor believes the Infinity Stones on Earth aren’t a priority because of the Avengers. There’s no way he could know the Guardians and the Avengers would eventually meet on Thanos’ home planet with the fate of the universe at stake. Plus, Thor hasn’t been with the Avengers since Age of Ultron. He wasn’t in Captain America: Civil War, and he’s pretty preoccupied with Hela in Thor: Ragnarok. When Mantis asks Thor if Kevin Bacon is an Avenger, he responds, “He may be on the team. I don’t know. I haven’t been there in a while.” At this point, Thor doesn’t even know Spider-Man. Yes, he could’ve told the Guardians more about Iron Man or Doctor Strange, but Thanos just killed half of his people. His mind is probably somewhere else.
Nevertheless, failing to share details about the Avengers is a mistake. Until now, Thor’s biggest blunder in Infinity War was supposedly when the God of Thunder hit Thanos with Stormbreaker in the chest rather than the head. The Mad Titan lived and eliminated half of all life with the snap of his fingers. But Doctor Strange revealed in Infinity War that there was only one way to defeat Thanos. The events at the end of Avengers: Infinity War and throughout Avengers: Endgame needed to play out exactly as they did. What didn’t need to happen was a fight between the Avengers and the Guardians of the Galaxy. That one’s on Thor.