Out of all of Earth’s mightiest heroes, there’s no one who can match the raw power of Thor. As a Norse god, he commands the power of the elements as well as Mjolnir, a hammer that can lay waste to whatever it touches. More often than not, Thor is the team’s bruiser; he goes in when there’s nothing left to do but fight. And even though he’s an all-around stand-up fellow, there are a lot of instances where he just does wretched stuff.
There are plenty of instances of Thor being manipulated or enchanted to commit evil acts, but some of the worst things Thor has done were under his own volition. For every time he was enchanted to fight his friends, there’s another story about the God of Thunder blowing China up or starting an international incident because he couldn't control his emotions. The worst things that Thor has ever done rank among some of the worst atrocities committed in the Marvel universe.
When Does It Happen: Thor #68
One of the worst-case scenarios that can happen to regular people from the Marvel comics universe is if a hero with the power of a god takes over the world. Thor #68 jumps forward to 2020 where the God of Thunder has taken over the world after a conflict with Asgard.
He joins forces with Loki, calls for the heads of the Avengers, and makes a rule that all newborns have to register in case they develop powers.
Thor goes fully Thanos and commits atrocities in the name of saving the world and gets rid of famine, conflict, and poverty. Everything gets reversed thanks to time travel shenanigans, but Thor remembers it all.
When Did It Happen: The Mighty Thor #159
In The Mighty Thor #159, Donald Blake (Thor's former secret identity) learns about his true nature from Odin. The All-Father explains to Blake that Thor willfully broke a treaty with the Niffleheim after it was signed by the Asgardians.
The two groups were meant to stay away from each other, but Thor walked into the land of the Niffleheim and started beating them with Mjolnir. He was pulled out of the area without any harm, but Odin decided to punish Thor by sending him to Earth to learn about humility until he was ready to be the God of Thunder.
When Did It Happen: Thor #52
Stories about Thor are quick to remind readers that, as a young god, he was impudent and didn't empathize with the humans who worshipped him.
Thor #52 tells the story of a young Thor encouraging the Viking slaying of thousands, only to realize the men who serve him aren't just offing each other, they're taking the lives of innocent people.
When Thor pays a visit to one of the war-torn areas, he discovers a loan orphan, Thialfi, left in a recently ravaged village. He begs Odin to do something, so Odin gives the child the gift of immortality as long as Thor promises to be his friend.
When Did It Happen: Invaders #32
It's clear that working with Hitler is as unheroic as it gets, but in Thor's defense, he didn't realize that he was working with one of history's worst monsters.
In this instance, Thor is brought from the past by one of Hitler's right-hand men (a young Victor Von Doom) and convinced to work for the SS to take out Joseph Stalin. Thor agrees to help, but when he realizes that Hitler and his followers are horrible people that plan to work with Trolls, he bails on them.