The Marvel Cinematic Universe is a powerhouse that slows no sign of slowing down. People have applauded the studio for making highly entertaining films that more or less faithfully adapt the spirit of the comic books in ways that are generally accessible to the everyday public. Most of the Marvel Superhero movies are based on the comic books, but not every storyline is adapted page for page. Marvel often finds it necessary to tweak and alter events so they play out differently in the films. Storytelling restraints, time management, and outdated ideas are all reasons why Marvel likes to change things up in ways that both fit the story and entertain audiences.
But sometimes, Marvel makes changes because the original comic book story was way too brutal to bring to the big screen. Marvel films mostly try to be a family-friendly venture, so events like the Red Skull mutilating his own face or Quicksilver and Scarlet Witch's incestuous relationship don't really fly on the silver screen. Below is a list of Marvel movie scenes vs. Marvel comic scenes that show how much Marvel has changed in their storylines.
Marvel has a tendency to make their heroes fight darker, mirror-versions of themselves in their films, and the first Captain America movie is no different. In Captain America: The First Avenger, Cap faces off with his arch-nemesis the Red Skull, who is revealed to be created by the same serum that turned Captain America from a shrimp into a beefcake. Although his rhetoric and demeanor matches more closely with the original character, his power level and origin matches more closely with the Ultimate version of the character. In that world, Steve Rogers has an illegitimate son who's taken and raised by the government. He has the same powers as Captain America and seems polite and kind, but on his 17th birthday, he reveals himself to be a true psychopath, butchering over 200 people and slicing up his own face in a way that makes him look like a Red Skull. This is way more brutal than anything Marvel would want in their films, especially when it was still trying to launch its cinematic universe.
This example may not be as brutal as others on this list, but it's pretty upsetting. When they were introduced in the film Avengers: Age of Ultron, Scarlet Witch and Quicksilver were shown as siblings who deeply cared for one another. In the comics, that relationship was taken to the extreme when one storyline had them become lovers as well as siblings. That's something you'll likely never see in the family-friendly Marvel Universe.
When Marvel made Ant-Man, they made a bold decision to not focus on the original version of the character, Hank Pym. Instead, they chose to focus on the lesser-known Scott Lang. Hank Pym was still in the film, played by Michael Douglas, but one of the most infamous moments from his comic history was unsurprisingly left out. Out of everything he has done, the comic book version of Pym is perhaps most remembered for hitting his wife Janet out of anger. It's been an unfortunate stigma on the character since it happened, and while the movie mentioned that Pym had a temper, it never suggested that he resorted to domestic abuse.
One of the biggest ways the Guardians of the Galaxy film differed from its comic was the depiction of the Kyln, the prison where all the characters were sent and started to work together. In the film, the prisoners seemed rowdy and dangerous, but there was never a clearly defined sense of danger.
The prison in the comics would eat the prison in the movies for breakfast. It's not only one of the most dangerous places in the galaxy, it also has a 99.999% inmate mortality rate - most don't survive more than three years. It doesn't sound like a place where a gigantic tree can just walk up and take a battery off a wall. Again, Marvel is in the business of making mostly family-friendly entertainment, so depicting a prison where people are brutally murdered left and right probably wouldn't fly.