Being a hero in the Marvel universe isn't an easy task. From battling criminals to world-conquering entities, these righteous champions give their lives to protect the innocent. However, one of the greatest enemies any hero can deal with is the darkness inside themselves.

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Whether it is villains swaying heroes to give in to their inner demons to powerful entities possessing their souls, some of the world's greatest protectors have turned into the universe's greatest foes. The change in character has resulted in some of Marvel's greatest stories, some even standing the test of time.

10 Speedball

Arc: Civil War: Front Lines

Speedball was one of the 90s most promising heroes, having a solo title and being part of the currently underrated New Warriors. In the 00s, the team was on the verge of uncovering a villain's secret hideout when they were discovered and had to take them down. The result was the villain Nitro unleashing an explosion that decimated Stamford, Connecticut, kickstarting the Civil War.

Being the only survivor, he blamed himself for the outcome and became Penance, an antihero seeking to undo the damage he caused. Fueled by his self-punishment, he worked with the Thunderbolts to capture unregistered heroes, much to the chagrin of his past New Warrior teammates. He learned to forgive himself and returned to his former persona, now looking to help train young heroes.

9 Archangel

Arc: Horsemen Of Apocalypse

Archangel moving in for the kill in Marvel Comics

Angel was one of the first mutants Professor X chose to be part of the X-Men, his wings being vital to the team. Unfortunately, during the Mutant Massacre storyline, Warren's wings were mutilated by the villain Harpoon, forcing them to be amputated to save his life. Warren sunk into depression, which made him the perfect target for the immortal mutant conqueror Apocalypse.

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Brainwashed and given blue skin and new metallic wings, Warren became Death, the leader of Apocalypse's Four Horsemen. Using his knowledge of his former teammates, he exploited their weaknesses while serving his new master. While he managed to break free of Apocalypse's thrall, he still struggles with his Death persona, his new wings serving as a reminder.

8 Bucky Barnes

Arc: Captain America: Out Of Time

Bucky Barnes The Winter Soldier

Bucky Barnes was the partner of Captain America, who was believed deceased by the end of the second World War. Years later, Captain America was on a mission to locate the stolen Cosmic Cube and when he came across a mysterious assassin who matched him in skill and strength. To his shock, Steve recognized him as his sidekick, now grown up.

In truth, Bucky's body was recovered by the Soviets and brainwashed to become the Winter Soldier, the perfect killing machine.

Now under the control of Cap's greatest enemy, the Red Skull, Bucky killed many innocent civilians. Though Steve managed to break Bucky from his mind control, he ventured alone to work things out.

7 Scarlet Witch

Arc: Avengers Disassembled/House Of M

Scarlet Witch uses the Darkhold in Marvel Comics

The Scarlet Witch is considered one of Marvel's strongest magic users, originally a villain before becoming part of the Avengers. However, she has faced various tragedies that nearly destroyed her psyche, from the death of her husband Vision to the revelation of her children being figments of her imagination. This all came to a head when her mind officially snapped, resulting in killing some of her Avengers teammates and disbanding the group.

Though she was caught, her brother Quicksilver manipulated her to reshape the world where Magneto ruled the planet, and she had her children back. Though her former friends stopped her, the outcome saw Wanda decrease the mutant population, escaping into isolation for a time.

6 Hulk

Arc: Future Imperfect

Maestro looking down in Marvel Comics

Alternate realities have shown fans the possibility of their favorite heroes turning villainous under the right circumstances. In one of these futures, the Hulk survives a nuclear holocaust, now bearing the intelligence of his alter-ego Bruce Banner. Matching brain with brawn, he renamed himself Maestro and seized control of the planet.

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To make his future permanent, he brought his younger self to his timeline in hopes of convincing Hulk to his side. When that failed, Maestro beat the Hulk to a pulp, proving himself the dominant monster. Though Hulk killed Maestro with a Gamma Bomb, his influence still haunts the Green Goliath.

5 Cyclops

Arc: The Avengers VS X-Men

Cyclops as the Dark Phoenix in Marvel Comics

Cyclops has been depicted as the reliable leader of the X-Men, a role model to his fellow teammates and other mutant civilians. Unfortunately, after certain events leading to the decimation of mutants across the globe, Cyclops lost his way, with his goal of restoring mutant kind bearing a close resemblance with Magneto.

When the Phoenix returned, Cyclops and a few other mutants became the hosts of its infinite power. Though he used the Phoenix's power to make the world a utopia, it began to corrupt the hero further, resulting in him killing his father figure Professor X. Though repentant for Xavier's death, he wouldn't be redeemed until 2019's House of X.

4 Daredevil

Arc: Shadowland

Daredevil Impales Bullseye Shadowland

After the fallout of Norman Osborn's Dark Reign, Marvel's craftiest fighter Daredevil saw him the leader of his former enemy, the Hand and dealing with the deaths of Osborn's agent Bullseye's victims in Hell's Kitchen. Bitter, he used the Hand to enforce his will in New York, unaware of a threat in the shadows.

His allies were rightfully worried about their friends, especially after he broke his no-killing rule to impale Bullseye out of vengeance. They learned that Matt was being controlled by the Beast of the Hand, an ancient demon whom the Hand worshiped. Iron Fist used his power to free Matt, though Matt temporarily retired to rediscover himself.

3 Iron Man

Arc: Superior Iron Man

Tony Stark as the Superior Iron Man

While Tony Stark has tried to use his technology to save lives, his ego, combined with his intelligence, hides the possible cruel man underneath. During the events of AXIS, several heroes' and villains' moral compasses were altered due to the Red Skull's tampering with Professor X's mind. While various heroes became villains, none were as dangerous as Iron Man.

Reverting to his old ways, Tony turned San Francisco into an advanced dystopia, infecting the cities' residents with the Extremis virus. Though his allies tried to stop him, Tony's proved too powerful to stop. In the end, both he and Captain America fought to the death, ending with both heroes dying, only to be brought back to their former selves in 2015's Secret Wars.

2 Ultimate Mr. Fantastic

Arc: Ultimate Enemy

The Maker taunts Eddie Brock in Venom

While the Ultimate universe shares similarities with its main 616 counterpart, there are a few unique differences. With several heroes dead and the world left in shambles after an attack by Magneto, The Fantastic Four broke up. Reed Richards was traumatized, due in part to Sue ending their engagement and Kang's revelation about the end of the world, his mind snapping in the process.

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Trying to prevent this, Reed's plans for global preservation saw him at odds with his former allies, resulting in his body becoming deformed. Now as the evil Maker, he became one of the Ultimate Universe's greatest threats, his intelligence and knowledge of his former friends making him a sinister mastermind.

1 Jean Grey

Arc: The Dark Phoenix Saga

Phoenix destroys a planet in the Dark Phoenix Saga

The story of the X-Men's first encounter with the Phoenix Force still ranks as one of Marvel's greatest tragedies. Jean Grey was one of the strongest and kindest X-Men members, serving as the group's moral compass. She eventually received a power boost thanks to the Phoenix Force, a power her friends feared and others craved.

Manipulated by the Hellfire Club, Jean transformed into the evil Dark Phoenix. Readers watched as the X-Men tried to save their friend, but even they couldn't stop her. She then went on a rampage across the universe, devouring planets to fuel her hunger. In the end, Jean sacrificed herself to prevent the Phoenix from destroying the galaxy, though fans knew this wouldn't be the end for the two.

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