It's not unusual for comic book characters to have multiple origin stories. Scarlet Witch's background, for example, underwent several retcons, and she's either a mutant or a chaos-powered individual, depending on the writer. The medium allows for unique takes on different characters, thus providing variety and dynamism to these colorful and ever-changing individuals.
When translated to the screen, comic book characters receive adjustments to make them seem more cinematic. This includes supervillains, whose origin stories usually change the most. Whether they're simplified or tied into other aspects of the overall plot, these characters' backstories might change, but they remain committed to their evil ways.
The Mandarin - Iron Man 3 (2013)
One of Marvel's most infamous and complex villains, the Mandarin is a genius scientist and skilled martial artist who debuted in the Silver Age of Comic Books. Iron Man's archenemy, the Mandarin is usually portrayed as a Chinese megalomaniac, heir to a generous fortune that's entirely spent on his education, who holds a grudge against his government, and eventually the entire world.
Seemingly played by Ben Kingsley in 2013's Iron Man 3, the character is eventually revealed to be the terrorist persona of a struggling actor, Trevor Slattery, acting under Aldrich Killian's command. This iteration received largely negative reviews from fans and the real Mandarin will debut in the Shang-Chi and the Legend of the Ten Rings, played by Chinese icon Tony Leung.
Baron Mordo - Doctor Strange (2016)
Arguably Doctor Strange's nemesis, Baron Mordo is a highly skilled magician who specialized in the black arts. Like his movie counterpart, Mordo studies magic under the Ancient One in Kamar-Taj. When Stephen Strange foils Mordo's plan to kill the Ancient One, thus becoming the old man's apprentice, Mordo is exiled by his former teacher.
The 2016 MCU movie Doctor Strange paints Mordo as a tragic villain. Gone is his plot to kill the Ancient One, replaced instead with a deep sense of loyalty towards his teacher. Most of the movie paints him in a positive light and is only at the end that he takes a darker path, although still nothing like his comic counterpart.
Baron Zemo - Captain America: Civil War (2016)
Going from one Marvel Baron to another, Helmut Zemo is one of several characters to don the Baron Zemo title. The 13th Baron Zemo, Helmut adopts the moniker to avenge the death of his father, Heinrich Zemo, a Nazi scientist who fought Captain America during WWII. The current Baron Zemo, Helmut is responsible for the reformation of the Masters of Evil and the formation of the Thunderbolts.
The Zemo in Captain America: Civil War is also looking to avenge the love of a dead one. His family perishes during Ultron's Sokovia attack and he plans to tear the Avengers apart by pitting Captain America and Iron Man against each other, eventually succeeding. He'll return in The Falcon and the Winter Soldier, and could maybe mimic his comic counterpart's action and bring the Thunderbolts to the MCU.
Bane - The Dark Knight Rises (2012)
The Man who Broke the Bat, Bane holds a place of honor in Batman's Rogues Gallery. A highly intelligent and skilled criminal, Bane spends years in the Peña Duro prison, serving his father's life sentence. Fluent in Latin, Spanish, English, and Portuguese, Bane is subjected to the Venom drug, which greatly enhances his strength and durability, but to which he becomes fully dependant. He infamously breaks Batman's back in the Knightfall storyline.
The Dark Knight Rises gives most of Bane's backstory to Talia al Ghul, reducing him to the role of her protector. He's still a strong and cunning adversary, but it's Talia who is the mastermind of the plot, with Bane acting as the muscle.
Vulture - Spider-Man: Homecoming (2017)
Brilliant but maniacal, Adrian Toomes turns to a life of crime after discovering his business partner embezzled money from their company. Growing more and more comfortable with his evil ways, he slowly becomes a heartless killer who constantly clashes with Spider-Man.
As they did with Mordo, the MCU turns Toomes into a more sympathetic character, an honest worker who turns to a life of crime after being taken advantage of by the system. Michael Keaton's Toomes is still intelligent, but the true power of his suit comes from the Chitauri technology he retrieves from the wreckage of the Battle of New York.
Cheetah - Wonder Woman 1984 (2020)
In the comics, Cheetah has multiple iterations, each with several origin stories. Barbara Ann Minerva, the Cheetah's third incarnation, is a beautiful, vain, selfish, and neurotic heiress and archeologist. Her powers and Cheetah form come from ingesting a potion made out of human blood and berries from the Urzkartaga region in Africa. Because the potion is meant for a virgin and Barbara isn't one, it leaves her in severe pain while in human form, and a constant feral state while in her Cheetah persona.
The Barbara in Wonder Woman 1984 is the polar opposite, similar to the character's depiction after DC Rebirth. Shy, ignored by potential suitors, and kind, Barbara is depicted as a lonely archeologist who befriends Diana. She wishes to be more like Diana, which grants her Wonder Woman's strengths. After wishing to be an apex predator, she becomes the Cheetah.
Mysterio - Spider-Man: Far From Home (2019)
Quentin Beck is a special effects artist and stunt man. After years of trying to make it in Hollywood, and with a failed acting career behind him, Beck decides to turn to a life of crime, using illusions to his advantage. During his first fight against Spider-Man, he frames the hero for robbing the Midtown Museum before getting jailed.
The MCU presents Beck as a resentful former Stark industries' scientist. He recruits other angry former Stark employees and employs drones equipped with holographic technology to create "Avengers-level threats" to turn himself into a hero.
Doomsday - Batman V. Superman: Dawn Of Justice (2016)
Born in prehistoric Krypton, Doomsday is the result of a cruel experiment performed by the alien scientist Bertron. Placed in a still harsh and unforgiving Krypton, baby Doomsday dies repeatedly, being cloned every time as a way of accelerated evolution. He retains the pain of all his deaths, driving him to hate all life forms. He eventually kills Bertrand and escapes Krypton, going on a rampage around the universe and eventually arriving at Earth, where he infamously kills Superman.
Batman v. Superman: Dawn of Justice features a Doomsday-like creature created by Lex Luthor from the remains of General Zod. The creature eventually kills Superman and perishes in the process, although it is revealed by the Kryptonian ship's computer that Krypton's council did something similar before, meaning another, more powerful Doomsday might be out in the universe.
Hela - Thor: Ragnarok (2017)
Based on the Norse goddess Hel, Marvel's Hela is the ruler of Hel and Niflheim. Born in Jötunheim as the child of a different incarnation of Loki and the giantess Angrboða, her attempts to take control over the death who dwell in Valhalla often put her in conflict with Odin and Thor.
The MCU turns Hela into Odin's eldest daughter and thus the rightful ruler of Asgard, unlike the comics where she is a full-blooded Jötunheimr giant. Bloodthirsty and ambitious, she assisted Odin in the conquering of the Nine Realms before being imprisoned due to her savage nature.
Joker - Batman (1989)
The Joker is a tricky character to adapt because he has no canon backstory. Instead, several major possibilities appear during the character's decades of appearances. The most common story features him falling to a tank of acid or chemical waste, resulting in his bleached skin, green hair, red lips, and eventual insanity. His backstory before this is unclear, though, with Alan Moore's The Killing Joke often considered to be the greatest Joker origin story ever.
Tim Burton's 1989 Batman mainly respects the character's accepted backstory and presents him as Jack Napier, a criminal who falls into a pool of acid, thus becoming the Joker. However, Burton majorly alters Batman's backstory by making Napier responsible for Thomas and Martha Wayne's murders. The change received a negative reception from comic book fans at the time, although the film's reputation and style remain highly regarded.
