The Marvel Cinematic Universe has offered up a strong mix of superheroes over its movies and Disney+ shows. However, if the MCU is anything like Marvel Comics, there is a chance that not all these heroes remain good guys. There are even hints spread throughout the movies and shows that some heroes might be ready to turn to the dark side.
There hasn't been much in the way of heroes turning dark yet in the MCU. The closest that Marvel came was in WandaVision with Scarlet Witch doing some unethical things before coming around in the end and in The Falcon and the Winter Soldier with Sharon Carter. However, there is still a chance that some heroes could go dark in the end.
Scarlet Witch
Wanda Maximoff pretty much went full villain in WandaVision. She mind controlled an entire town and forced the people there to unwillingly live in a fantasy world. She even went to battle with Monica Rambeau and Vision to keep her fake world alive.
However, that isn't the worst thing that Scarlet Witch ever did in comics. Wanda has lost her entire grip on reality more than once and she actually killed two heroes in Scott Lang's Ant-Man and Hawkeye during one moment, as well as wiping out mutants. Wanda was last seen doing questionably ethical things in WandaVision and could easily turn evil in the end.
Yelena Belova
Yelena Belova looked like she was going to turn evil in the post-credits scene from Black Widow. It was at that moment when Contessa Valentina Allegra de Fontaine tasked her with killing Hawkeye, lying that he was responsible for Black Widow's death.
This would have crushed many fans, as Yelena had qualities that made her a fan favorite in Black Widow. She ended up not killing Hawkeye and lived as a hero for one more day, but she is still an assassin, so that puts her on a line where she is one step away from becoming a villain in the MCU.
Captain America
Captain America ended Avengers: Endgame by making his own sacrifice. While Iron Man died to save the world, Steve Rogers gave up his role as a hero to go back in time and reconnect with Peggy Carter and he grew old with the woman he loved. It seemed like the perfect happy ending.
The problem is that the MCU has not revealed what happened to Steve Rogers. It might be a hard sell, but there is always a chance Steve is still around and if Marvel wanted to bring a version of Secret Empire to the big screen, Steve could become the biggest villain on Earth in a story that might hit a little too close to home for some people.
Hulk
Hulk is still out there and he is rumored to return in She-Hulk on Disney+ when that series arrives. The last time anyone saw him, Hulk was smart and he seemed to share the same brainwave as Bruce Banner, with the latter even appearing in the Shang Chi post-credits scene.
However, there is a thought that Hulk could end up losing control again and that it would take the world of heroes to bring him down. Whether this means World War Hulk or a twist on the Maestro storyline, Hulk turning bad is something that could very much happen in the MCU.
Nick Fury
Nick Fury has seemed to be a man with unlimited lives. Just when everyone thought he died in Captain America: The Winter Soldier, he returned and admitted he faked his death. Then, he appeared in Spider-Man: Far From Home only for it to reveal he was a Skrull in disguise.
It turned out the real Nick Fury was in outer space with the Skrulls. But, what if that isn't the real Nick Fury? What if Nick is dead and the Fury in space is a Skrull planning out the upcoming attack coming in the Disney+ series Secret Invasion? Nick Fury could lead the attack on Earth.
War Machine
War Machine has his own Disney+ series coming called Armor Wars. For non-comic book fans, this is based on a classic Iron Man series where Tony Stark set out to retrieve all the armor that was made by Stark International. The problem is that he decided to take everything back.
Stark believed all Stark tech was his and he attacked even prisons and stole the guards armor and attacked other Marvel heroes to take their armor. Even Captain America believed that Iron Man was bordering on villainous actions and if War Machine does the same it will be worse since it isn't his tech he is trying to get back.
Doctor Strange
The MCU has already shown how Doctor Strange could turn evil in the movies. This happened in the What If...? series on Disney+ and the trailers for Doctor Strange in the Multiverse of Madness hints that could very well happen in live-action format.
The trailer made it look like the evil Doctor Strange was going to show up in the movie. Strange has already done several things that might not be that moral, including almost breaking the multiverse. With a new Sorcerer Supreme, Strange is one bad decision away from being the next major threat in the MCU.
Gamora
The idea of Gamora turning evil is closer than many Guardians of the Galaxy fans might want to believe. This fact is because Gamora is not the same character that MCU fans grew to love over the first two movies. That Gamora died at the hands of Thanos.
The Gamora in the MCU now is an alternate version from the multiverse, one that never went against Thanos until the final fight in Endgame. In the upcoming Guardians of the Galaxy Vol. 3, there is a good chance that Nebula will be a much more heroic character than the new Gamora.
Hank Pym
Another hero that turned into a villain in What If...? was Hank Pym. He killed most of the Avengers before the team could form and it took Nick Fury and Loki to stop him. Pym was always a morally grey character in the comics, and he has been the same in the movies.
While Hank Pym seemed to have his happy ending, getting Janet Van Dyne back in his life, that does not mean his story is finished. While Kang is coming in Ant-Man and the Wasp: Quantumania, there is always a chance Hank Pym could finally go full bad and betray Scott Lang to side with the conqueror.
Miek
There is a chance Hulk could turn bad and either lead the way to World War Hulk or to a Maestro storyline. If Bruce Banner loses control again, it would likely take a terrible tragedy to finally bury him behind the rage once and for all. The World War Hulk storyline could show how it could happen.
In that storyline, a bomb killed Hulk's wife and unborn baby and he went on a rampage, seeking revenge against all Earth's heroes. Miek set the bomb off to force Hulk to become a warrior once again. Miek is still alive and living with Korg, and if he does something to force Hulk into battle, it could destroy everything in the Green Goliath's life and turn the loveable alien into a truly horrific monster.