Not all stories are "black and white".
And not all "black and white" stories are 100% "black and white".
However, people used to a "this side is right, this side is wrong" often are unable to accept that.
As such, this trope kicks in.
This is both an audience trope and an adaptation trope. Namely, when either the audience or the authors remove the complexity of an issue and act like one side is unquestionably right or one side is unquestionably wrong.
This is different from the Informed Wrongness , Designated Hero/Designated Villain and Strawman Has a Point tropes in that the original work IS nuanced, it's either audiences or a later adaptation (or both) who completely remove the nuance from the work.
So, to clarify the trope name: this is when people take an issue that wasn't originally "black and white" and make it into "black and white". They find something (a work) that is not black (that doesn't run on Black and White Morality) and paint it black (change it so it does run on Black and White Morality)
When the opposite happens (audiences and/or later adaptations take a work that is "black and white" and try to make it "grey"), it's Golden Mean Conflict.
- In the original comics, the Kree-Skrull War was a morally grey conflict with no "right" side (both the Kree and the Skrull are depicted as antagonists to the superheroes, making the whole conflict come across as Evil vs. Evil). In Captain Marvel, this is replaced by a "Kree bad, Skrull good" narrative.
- Westeros is a Crapsack World, and everyone in it has at least some level of evil (or at least, Jerkass). This doesn't stop audiences from perceiving Jon Snow as "the hero" of A Song of Ice and Fire/Game of Thrones , even though Jon is as corrupted as everyone else.
- The original V for Vendetta comic book portrayed fascism and anarchy as two undesirable extremes, but the movie goes for a simpler narrative of "heroic V saves England from the tyranny of Norsefire".
- The original Men in Black comics have the titular Men in Black commit as much atrocities as the aliens they hunt, and the only reason they got into the alien-hunting business is so they would have no competition when the time came for them to Take Over the World . The movies simplify this into "Men in Black good, aliens bad".
- Notably, even supposedly "truthful" adaptations of Frankenstein will either make Frankenstein's Monster the "good guy" and Victor Frankenstein "the bad guy", or Victor "the good guy" and the Monster "the bad guy". In the book, both Victor and the Monster are just as flawed.
- The whole "why doesn't Superman kill the Joker?" issue is a complex one with no real "right" answer, but both writers and fans either simplify it into "Superman instantly goes evil the moment he kills the Joker" or "Superman does the world a favor by killing the Joker".