Kentaro Miura’s Berserk. The setting’s one where evil is not only basically everywhere, but is a literal conceptual power-up for those who commit to it. Because of humanity’s selfishness leading them to always desire to have someone or something else to blame for their troubles instead of accepting their own faults and facing them head on to grow as people, they literally created a God of Evil from their collective consciousness.
This God, being the unspecific “someone” who all humans at one point or another desire to blame for their own faults or suffering, is not just a severely blamed guy like Angra Mainyu from fate who hadn’t committed any crimes, IT ACTUALLY IS responsible for virtually every single evil that happens in the world. It can control and see causality itself, giving near-omnipotent control of the patterns of events that make up life, meaning it can decide the fate of someone to be a horrific death from birth, and that someone has virtually no chance of actually doing anything about changing that fate no matter how hard they try. And because it was created from the desire to have someone responsible for every single evil that happens in the world, it uses its immense power to make sure that patterns of events, no matter who is involved in them, no matter what place or time, will almost always go in a way that results in evil is fated to win over good. This means that most people in the world are selfish to the extreme and all carry a bit of sadism, making rape and cannibalism of even children quite frequent.
In addition, it alters events so that when certain humans are driven to absolute despair and crushing defeat, they always, no matter what, find artifacts that allow them to gain the conceptual power of evil itself to turn from humans into demons... by sacrificing everyone they care about to gain the power. Even worse, the God is responsible for them being driven to these zeniths of despair in the first place by its power to manipulate causality to force them down the road of misfortune. Once they die, they are dragged into hell for their crimes, yes, but so is every single person who was ever involved with them and died, no matter how innocent those people were, even if they were the ones cruelly sacrificed for power. The demons themselves are so powerful physically and sometimes magically that even the weakest of them can take some of the strongest humans and win easily.
Even the “hero” of the story is near-sociopathic and bloodthirsty as all hell, and only does good deeds if they help him get what he wants. Virtually every noble in existence at some point is sadistic and greedy for more power, torturing and killing the poor at their whim or if they have something they want. The world is near constantly at war between the various empires and kingdoms, and while temporary peaces sometimes happen, the wars will ALWAYS repeat because the God of Evil ensures that malicious people will always be the ones guiding the next generations, leading to generational absolute hatred of the other side. Diplomacy is a joke, and the kingdoms themselves are corrupt to the core. The king of the protagonist’s nation was so utterly pathetic as a ruler and father that when his own daughter had her first time with someone, his response was to torture her boyfriend and rape her himself. The Mughal-Empire analogue in the setting’s emperor is a man who was tortured by his own stepmother in her bid for power while growing up, and became so bitter and hateful towards the world that in order to make himself more powerful when he finally achieved becoming emperor, he made a literal tower of the conceptual evil artifacts of the God who rules causality. Then he had his own citizens raped by monsters in order to create servant beasts from their corpses, and turned himself into a literal avatar of destruction against his enemies but also his own people using the tower of evil. The king of Falconia, arguably the most beautiful kingdom city in the world, a man who seems more benevolent than most rulers, is in fact a manipulative, psychopathic Demon Lord who sacrificed his entire army of friends who were loyal to him at his most despair-filled moment to ascend higher as a demon than most, and the main antagonist. Even worse, he is practically invulnerable to conventional and magical weaponry and charismatic in the extreme to the point that some of the few genuinely good people left in the world think he’s a hero.
For another setting, Craig Halloran’s Darkslayer. While this setting is nowhere near as dark as Berserk, considering the good guys do manage to win quite often in the end, it is still fairly bleak. The universe is filled with god-like beings that have reality warping powers that can shape planets themselves to their bidding and create life, and while some of these beings are seemingly very much malevolent or benevolent, their actual moralities are more similar to blue and orange, completely incomprehensible to humans if actually seen the truth of. The planet the series takes place on luckily has the creator god-being mostly ignoring it, but later on it is not nearly so lucky as it becomes the site of a years-long battle between two god-beings taking human-like forms with limited warping abilities in a beat, causing calamity and nearly annihilating the planet.
The planet was already bad enough before the god-beings directly went down to it to fight, though. It is divided into two sides of a coin. The first side is one where humanity and other sentient races are barely hanging on. The world is virtually completely filled with dangerous areas and monsters to the point where the planet is so uninhabitable that only five large cities exist. Most small villages are constantly at risk of being wiped out by monsters who are far stronger than normal humans, but the cities aren’t much better. There is so much corruption among the nobility of the various cities that barely any actual good nobles exist anymore because they’ve been either been killed in power plays or lost all their influence because they refuse to sink to the levels of the corrupt ones, which means that the poor are generally tormented or killed if they annoy the current nobility at all, or even just to amuse them. There’s also, of course, racism between various races to the point that anyone with mutations out of the norm for one is called a demon. Then there’s the Underlings, the actual demons.
The Underlings are a race of creatures that live in a hellish city underground, who are quite literally evil incarnate. It’s not that they are a people who in general choose to do evil far more than good, it is that they have no ability to do good at all. They were created by the god-being who shaped the planet to be the absolute evil of the world, a race whose members only exist to manipulate, kill, and torture other races and their own for their own individual growth in wealth and influence and power and sadistic pleasure. Even worse, they are actually great at this since they were designed to have no moral compunctions and have existed for thousands of years. Then there’s the other side of the coin-planet.
On the other side of the planet, things are even worse. There aren’t any humans or other races like elves, dwarves, or such here, and there is none of the absolute evil known as the Underlings. So why is it so bad? There is an entire race of literal cannibal giants there, of which only one is a good person. The rest are people who actively take part in warping space-time to cross the coin’s side in raids to capture and eat people of the smaller races from the other face of the planet, despite knowing for a fact that they, like the giants, were also sentient creatures. Then there’s the anti-heroic black dragon who serves as their watchdog.