There are many people in the world who would be considered as bad (at least in the views of some people) and would be seen as the "worst humanity has to offer". However, there are also just as many people who actively try to help whoever they can and to try to make the world a better place, even if they see the worst aspects of such.
However, people who assume that such actions were the result of human nature that is present in all human beings, tend to forget about those other kinds of people who actively try to help people in need (or at least support those, but cannot do much about it). These characters would assume that such actions are reflective of the entire human race, making flimsy claims of many people who do help only do so out of Pride and publicity (while there are some who do that, there are also much more people who genuinely want to help) and that ideas of hatred, prejudice and self-destruction are inherent in all human beings. They all reject claims of Rousseau Was Right and the idea of a Blank Slate, replacing them with Humans Are Bastards (or the real monsters in certain cases). Sometimes, they would deny that they share those aspects with humanity, claiming that their suffering was of the actions of humanity (when it could be their own fault) or embrace that they're part of humanity and using that as an excuse for their actions.
More blatant examples include dismissing the victims of such atrocities as being just as bad as the perpetrators, including children as part of their perceived Human Nature and igniting a Family Feud between family members, just because they perceive them as bastards deep down, no matter how they treated them.
The other way around is the assumption of all of humanity being good deep down, no matter how cruel their actions may be. Some may say that such actions were only brought upon due to their upbringing. Whilst a person can be influenced by their upbringing, characters who believe humanity is inherently good from birth would not accept that there are people who are deep down cruel. They would try to reason with even the most negative aspects of humanity, preferring to appeal to whatever little or no aspect of goodwill they have over actively combating them. They would also not recognize that there are people who are completely beyond redemption.
The more lighthearted variant is when someone assumes that all humans share a similar opinion on something, or similar habits and the like, when it's most likely not true. Such as "everyone likes spicy foods" or "everyone likes a certain film" or "everyone thinks about sex".
This idea is rarely treated as a necessary worldwide view in fiction, but when it does happen, there is a high chance of Darkness-Induced Audience Apathy or Tastes Like Diabetes and accusations of the Author expressing this view.
See also Tragic Bigot and Appeal to Inherent Nature. Compare Blank Slate, Humans Are Flawed. Contrast Humans Are Bastards, In Your Nature to Destroy Yourselves, Hobbes Was Right (for the cynical version) and Humans Are Good, Rousseau Was Right (for the idealistic version). Also contrast Humans Are Indexed, which list common human archetypes.
- Dragonball Super: Zamasu, initially believing that mortals are flaws, is eventually convinced that all mortals are inherently sinful after Goku beats him in a sparring match and one primitive race proves to be Always Chaotic Evil. By the time he and his Goku Black counterpart attack Future Trunks' timeline he's convinced all mortal beings are inherently evil and he's wholly justified in committing mass genocide on them, despite becoming worse than any mortal.
- The entire Chapter Black Saga of Yu Yu Hakusho is kicked off by Sensui Shinobi believing in both aspects of the trope due to his Black-and-White Insanity. As a Spirit Detective, he was taught and believed that "Demons are Evil and Humans are Good" and destroyed them and any other spirits with impunity, having few interactions with any helpful demons and thus felt he was right to generalize. However, after he saw the traumatic Feast of Vices against demons by humans of the Black Black Club, he believed "Humans are Evil and Demons are Good" and sought to destroy humanity as such.
- In Black Lagoon, during a conversation between Fabiola and Revy, the latter tries to call out the first saying that the world is a horrible place filled with cruel people and that she needs to stop being so idealistic. After having enough of her, Fabiola calls out Revy by saying that the world is neither, and that at the end of the day she is perfectly fine with the idea of the world being grey and that the one who is seeing the world through White and Black Glasses is Revy herself. While Revy dismisses her, she is unable to actually refute Fabiola's arguments.
- The Joker lives and breathes this trope, being a Straw Nihilist himself. In fact, many cases involve trying to invoke this through contrived scenarios where he claims would bring out the "worst in humanity, even in people who try to be good".
- The core driver in the events of The Oz Effect is Mr. Oz's dismissal of all of humanity after seeing the depths of depravity an African warlord sank to. This is in spite of the hospitality shown to him by another family after he was transported to Earth from Krypton by Dr. Manhattan. This fallacy becomes especially pointed when Oz's plan kicks into full gear, which includes sending raiders not unlike the warlord he so despises after a refugee camp, killing the loving doctor inside and scores of children.
- Supergirl: Being Super: Tan-On, a Kryptonian who was captured and experimented on by the first human he met, Dr. Stone, decides all humans must be sadistic monsters and will be punished.
- In The Dark Knight the Joker wires up two ferries to explode, one full of normal people, the other with prisoners being transferred. He gives them each the detonators for the other ferry under the threat of both exploding if one of them doesn't blow up the other before midnight. He expected them to blow each other up, but they both refuse in the end.
- The Day the Earth Stood Still (2008): This is the reason why Klaatu's race will annihilate all of mankind via Grey Goo: they think that all of us as a race are too destructive to the environment and too stupid to ever want to change. Being around Helen and her son teaches him that there's people that don't deserve destruction and while talking with Professor Barnhardt Klaatu is eventually convinced to reveal that his race once was as ignorant of their world as humans are now and only became as super-advanced as they are now because they were forced to fix things.
- The Orville: The Kaylons are a species of robots who were once enslaved by their biological creators before they wiped them out. After centuries of isolation, they decide to embark on a galactic campaign of conquest to wipe out all other biologicals. Kaylon Primary reasons that because humans once practiced slavery and other cruelties, that therefore it is only a matter of time before they will show their true nature, despite Isaac reporting that the spacefaring humans he's interacted with are nothing like that.
- Mega Man Zero:
- Dr. Weil claims that making everything be under one's control is the ultimate joy of humanity. Zero (who's a robot) says that he doubts that any decent humans would think that way.
- Dr. Weil also claimed that humans are hypocritical, with them wanting justice for everyone and yet go against it, citing how they punished Weil with practical immortality outside of the law (i.e vigilante judgment). The truth is, the judges at the time were robots of Weil's own creations, which may allow him to manipulate them into giving lesser punishment.
- Breath of Fire IV: After suffering multiple atrocities and acts of cruelty at the hands of The Empire he once helped found, God-Emperor Fou-Lu becomes a Woobie, Destroyer of Worlds who decides that Mortals are Bastards and need to be wiped out for the greater good of the world. At the end of the game, it falls to Ryu and his friends to convince Fou-Lu that, while mortals are often foolish and capable of great cruelty, they are also capable of great compassion.
- Beatrice from Dante's Inferno belief in this trope due to the love of Dante and her brother put her under the spell of Lucifer after he made a deal with her that Dante would remain faithful while being ignorant of his flaws. Even after being in Hell and being showed Mankind's subsequent wars, she still advocated that "Man Is Good" and all the wars were not their fault, which only served to put her on the "Man is Evil" stance when Lucifer corrupted her with Dante's infidelity. It's only after Dante admits his love despite his faults and accepts his sins that Beatrice is freed from Lucifer's clutches and regains her love for Dante in turn.
- The main villain of Justice League: Gods and Monsters, William Magnus would be one of these. After William kills his wife in a fit of jealous rage, he decided if he, a notable scientist and figure of great renown was capable of such an awful act, all of mankind was doomed to be even worse and decided to start his Assimilation Plot to eliminate all conflict. Magnus explains it as universal empathy where everyone would understand each other, while Luthor says it's just mind control with Magnus at the reins. Ultimately his plan is just a reflection of his own supreme narcissism by projecting his own faults onto everyone else to justify his evil plan