Warning: This article contains MAJOR spoilers for Doctor Strange in the Multiverse of Madness.
With the recent release of Doctor Strange in the Multiverse of Madness, Marvel fans are getting a chance to see not only new superheroes, but also some new, and even some old, villains. The MCU is notorious for creating characters that start off as heroes and end up turning to the bad side, or vice versa.
Over the 14 years that the MCU has been coming out with content, there have been many characters that switch sides at one point or another. Some great ones start out as heroes or are intended to be that way, only to become villains down the line.
Ultron
Bruce Banner and Tony Stark worked together to try and create a Peacekeeping Program from the Mind Stone that was in Loki's scepter, but that went wrong when Ultron came to the realization that humanity was the biggest threat to peace on Earth. Ultron was intended for good, but when he gained consciousness and uploaded himself to a body, things went south quickly.
The Avengers had to quickly assemble to defeat him before he destroyed the Earth. Getting rid of Ultron wasn't an easy decision for everyone on the team since he was initially created for a good reason, but in the end, he had to be defeated. The good he was intended for ended up resulting in the form of Vision who was the one that killed Ultron for good.
Doctor Strange
Similar to Loki, Doctor Strange is one of the characters in the MCU that doesn't exactly seem to pick a side, at least not right away. It takes him a while to accept the sorcerers and their mystic arts, and even once he does, he still has the tendency to do things for his own personal gain. He also wasn't thrilled about joining the Avengers.
Strange is never fully committed to the side of the villains though he does go there. There are many signature Strange traits in the MCU, and his stubbornness and power are just a few of those. While he breaks the rules and makes some questionable decisions, it's never entirely a bad thing. He might not have been a straight villain in Spider-Man: No Way Home but he was an antagonist.
Baron Mordo
Once a friend of Stephen Strange and an ally of the Ancient One, Mordo turned at the end of Doctor Strange after the defeat of Dormammu. Due to the Ancient One and Strange both breaking rules that the sorcerers were supposed to follow, he began to become confused and left the order, condemning magic.
At the end of the film, viewers see him visiting a man who learned to use the magic to allow him to walk again. He strips him of his powers and vows to continue doing that to every sorcerer until he is the only one left in the world. His path wasn't entirely clarified in the second movie, as fans only see him in another multiverse.
Hawkeye/Ronin
Hawkeye is one of the original Avengers, but things turned for the worse for him when Thanos snapped his fingers. Not only was he not really aware of the whole Thanos situation since he didn't fight with the Avengers during all that due to the toll the battle in Civil War took on him and his family, but he also lost his entire immediate family during The Blip.
This much loss caused him to become angry and take on the role of Ronin. He went around and exacted vengeance on criminals that survived The Blip. Ronin went on a murder spree and developed quite the name for himself that followed him even after he retired the character, as fans see in Hawkeye.
Norman Osborn
Fans were thrilled to see the return of many classic Spider-Man characters in Spider-Man: No Way Home. Green Goblin was an especially big hit for fans, and when he sided with Peter Parker to create a cure for each of the villains it seemed like a heartfelt moment. When Peter sensed his betrayal it was heartbreaking, but also one of the best Peter tingle moments in all the Spider-Man movies.
Green Goblin really turned Osborn for the worst as he ended up being the one who killed Aunt May. While Osborn may have not been the one to blame, his alter-ego ended up making him a villain time and time again.
Loki
The God of Mischief, Loki, might be the original hero turned villain in the MCU. Throughout his entire time in the MCU, which side he is playing for is never entirely clear, except that he always likes to play in whatever way favors him. As he constantly stabbed his brother, Thor, his family, his friends, and his enemies in the back, Loki was often seen as the bad guy, including in The Avengers.
As much as Loki played the villain, he also came to the rescue and could help out at times. He wasn't entirely hated by Thor and the rest of the Avengers, but he wasn't exactly considered a hero. The tricks he liked to play are what prevented him from ever entering the hero ranks officially.
Captain America - John Walker
Disney+'s Falcon and the Winter Soldier introduced fans to the newest Captain America, which was taken on by John Walker. Walker appeared to be the perfect fit for the role on paper. He had received three Medals of Honor and had a much more extensive military career. However, Walker took his anger out on one of the Flag Smashers after Karli Morgenthau killed his best friend Lemar Hoskins.
Dressed in his Captain America suit, he murdered a man using the shield in front of lots of civilians, tainting Captain America's name, as well as his own. After he was stripped of the title, he became the U.S. Agent, and the costume was a darker look-a-like than Captain Americas's outfit. Both the Captain America suit and the U.S. Agent suits are ranked as the most accurate costumes from comics.
Bucky Barnes
Just like Steve Rodgers, Bucky Barnes was just a kid from Brooklyn. He fought in World War II and ended up joining Rodgers' group of men to fight against Hydra. On a mission that went wrong, Barnes was seen falling to his death. Hydra found his body and turned him into the Winter Soldier.
The Winter Soldier killed many ruthlessly, just as he was programmed to do. It was a lot for him to recover from once Steve found him again and tried to revive his friend. Similar to Hawkeye, he started off as a hero, was turned into a villain, entirely against his will, and then became a hero again, giving him quite a character arc.
Ikaris
Ultimately, Eternals may be the fresh start to the MCU that fans needed, for a variety of reasons. Dividing the team up and having the villain emerge from that team is one of them. Ikaris did all he could to keep his fellow Eternals on the path to complete Arishem's plan.
He went so far as to fight against his own brothers and sisters to make sure that this didn't happen. While he didn't prevail in his mission, he realized that what he had done was run and ended up flying into the sun, fulfilling the legend his name brought.
Scarlet Witch
Another Marvel character that switched sides a lot was Wanda. She started off bad, then turned good, but after Vision died, it was very unclear whether she was a hero or a villain. One thing was clear - she just wanted to live with Vis and her kids and have a normal life.
In Doctor Strange in the Multiverse of Madness, Wanda shows her true colors and fully embraces her witch powers becoming the Scarlet Witch. She attacks Strange, Wong, America Chavez, and many others just to get what she wants so she can be with her boys. She might be the most brutal villain in the entire MCU.