"If a man dedicates his life to good deeds and the welfare of others, he will die unthanked and unremembered. If he exercises his genius bringing misery and death to billions, his name will echo down through the millennia for a hundred lifetimes. Infamy is always more preferable to ignominy."
— Fabius Bile at the Desecration of Kanzuz IX Source: Codex: Chaos Space Marines (4th Edition)
Battle Machines: Carman Skiric destroys communities on planets he wants to capture because, in his eyes, people exist only as a resource, somewhere below energy and metal. The only reason the Earth and Sol governments have him as the commander of their armies is that he is so utterly terrifying that many foes just give up, rather than have him come down and fight. He's also ax crazy and his personal mech wields four chainsaws, he uses them because, quote, "I like to see the expression of dawning realisation on their faces, right before I tear them apart".
Chronologically speaking, first there's Stefan Amaris, the Usurper. He's the lovely bundle of joy who overthrew the Star League, executed everyone in the Star League Court, killed everyone in the Vatican before burning the place down, and a whole laundry list of deeds including genocide and sterilizing planets.
Katherine Steiner-Davion was arguably a fine politician when first presented, but a combination of author fiat and other factors led up to the revelation that she'd hired an assassin to kill her own mother, her brother, her other brother's girlfriend, and her own boyfriend. All for political gain. After the rest of humanity started catching on to her game, she became the focal point of a civil war that wrecked both of the nations she ruled, sold out the nation she had the most loyalty with to the Clans (in an attempt to kill her brother and some mercenary outfits that weren't on her team) and finally - after being deposed - escaped with her Clan allies and created a test tube baby with her rival brother's DNA and her own who would hopefully go on to cause more havok in the Inner Sphere at a later date.
Even her aforementioned son couldn't take it. He ends up killing her after she advocates murdering an ally, to protect her own position. Especially since maintaining her position may get him and his Clan destroyed.
An even earlier example is Star League General Amos Forlough, who helped conquer the Periphery during the Reunification War. While his actions against the Taurian Concordat, which included saturation level orbital bombardment and WMD deployment, may have been justified due to the fact that they were doing the same things; his actions on the Outworlds Alliance front went way too far. The Alliance barely had a regular military and so they used irregular militia to fight back with bombings and sniper attacks against his troops. He responded by killing 10% of the population of every planet that resisted (roughly 12 million people across the Outworlds), left many more without homes or shelter and sentenced his own subordinates to death and hard labor if they refused to carry out these atrocities. Little wonder why he is known as "The Butcher" and "The Baby Killer". The worst part is that he was never brought to justice and received some of the Star League's highest rewards for the victories he achieved.
Deadlands: Jasper Stone is the Harrowed responsible for bringing Hell to earth. Originally a Confederate officer with a reputation for cruelty and brutality. When he became the servant of the Reckoners, the Four Horsemen of the Apocaypse, Stone ruthlessly killed any who could stand in their way, hunting down and murdering countless heroes. Stone does everything of his own free will for his hunger to take lives and to see hell on earth enacted.
Dragonstar: Mezzenbone, an ancient and powerful red wyrm, is The Emperor of the Dragon Empire, the first chromatic dragon to hold that position after five millennia of good metallic emperors. Like most other reds he's cruel and regards the "lesser races" as cattle at best, but even among his Always Chaotic Evil kin he stands out for both his deranged mentality and catastrophic plans for the galaxy. During the great Dragon War between chromatic and metallic dragons that cost billions of lives and left countless worlds devastated in its wake Mezzenbone was first an important wyrm in the chromatic Asamet Kingdom and then its leader after the first king died under unclear circumstances that he may or may not have orchestrated, and while most rational beings abhorred the enormous tragedy of the War Mezzenbone, as a Blood Knight in the service of the Destroyer, reveled in the slaughter, planning not only the annihilation of his enemies but also that of his own kind and setting himself up to rule the scorched remains of a lifeless universe as a god. He only changed his mind when he realized the War would claim his own life as well if it escalated further, and agreed to the truce that would form the Dragon Empire only because he knew he needed the 5000 years until his guaranteed ascension to the throne in order to ensure his apocalyptic scheme would unfold flawlessly. In the present day of the setting, forty years after his coronation, he has introduced a number of controversial acts including forming an evil Secret Police of dark elves, supporting open worship of his patron deity, and vastly increasing the rate at which his armies conquer and assimilate new worlds into the Empire, all secretly for the purpose of finally bringing destruction to all that is not him. When he believes the time is right, Mezzenbone will unleash everything at his disposal (which includes systems-destroying superweapons and magical artifacts of untold power) and in a matter of minutes completely shatter all possible opposition to his scheduled omnicide.
Curtis Boatman, one-man reason that the Architects of the Flesh are unplayable, is the source of basically everything wrong with the 2056 juncture that isn't his ostensible boss's fault, from Abominations to the surveillance state. Unlike his boss, he is about as far from sympathetic as possible-he's described in-game as a bloated, corrupt, and utterly selfish jerk whose own motive is his own profit, and is personally responsible for several sadistic experiments. His laundry list of crimes included the Uber-kids, tricking normal people into accepting The Corruption of Arcanotech and then accelerating it deliberately so he can have perfectly loyal Super Soldiers, keeps an entire greenhousse complex of Black Market Produce for himself (even his boss, Bonengel, puts his money where his mouth is and buys real food honestly), plans to betray Bonengel because Boatman likes the Crapsack World with himself at the top of the pyramid as it is (and Bonengel had a Heel Realization a few years before and has become Necessarily Evil), and despite the Buro wanting equality at all costs, he's a huge bigot responsible for the rampant homophobia in the 2056 juncture. Most damming of all is the Fisher Kingdom aspect of chi mastery over a juncture-2056 under Architect rulership is his ideal world. Look at the utterly dystopian future and realize what that says about him.
Iron Kingdoms: Toruk is the Monster Progenitor of the dragon race, although he considers his progeny the only thing that might pose a threat to him and desires to destroy them all. To accomplish this he becomes an Omnicidal Maniac against every other race. He turns a whole set of island nations into a mix of undead and mutants to serve as his army, all with the intent of slaughtering as many of the inhabitants of the mainland as possible to make a bigger army.
Magic: The Gathering: Yawgmoth was born a normal Thran on the world of Dominaria during the reign of the technologically advanced Thran empire, about 5000 years before the birth of the inventor Urza. He was banished for his unorthodox beliefs regarding diseases and healing, but returned to the capital city of Halcyon to treat Glacian, the renowned engineer and inventor, who was suffering from an unknown illness after being attacked by an exiled leper named Gix. Though Yawgmoth was by trade & profession a medic, his ways went towards an unnatural fascination with the mechanics of the body. This fascination led to experiments with plagues and poisons, several of which caused widespread death among the various races that he had visited. Yawgmoth was also slowly drianing Glacian's soul to fuel his own power. Yawgmoth later learned about planes from the wizard Dyfed, until he subdued her and gave her to his followers for dissection. Ruing his own realm, Yawgmoth plotted further conquest, corrupting and killing other heroes and innocents. When he invaded Dominaria, he killed millions of innocents before he was destroyed.
Wilhelm Kantor, alias Overshadow, is a Nazi warcriminal and Evil Sorcerer turned international terrorist. As a member of the SS and the Thule Society, Kantor learned that he was the reincarnation of Tan-Aktor, a Treacherous Advisor to the Egyptian Pharoah Heru-Ra. Sacrificing twelve of his men, Kantor regained his past memories and powers, then went onto become the true mastermind behind the Nazi Ubersoldaten program. When Schwartzpanzer was crippled in combat, Kantor sacrificed him to increase the power of his own personal assassin, Nacht-Krieger; when the war ended Kantor would use the ritual suicides of his SS and Thule Society colleagues to further empower himself and Nacht-Krieger. Kantor went onto kill almost the entire roster of the Allies of Freedom, left Nacht-Krieger to take the fall, and set up the terrorist group SHADOW. He has since attacked world capitals with armies of clones, tried to get the USA and USSR to nuke one another, poisoned municipal water supplies, and attempted to use Omega's power to take control of space/time. In the meantime, he has kept himself alive, first by bodyjacking clones of himself, and later trying to take over the body of his own son, Ragnarok. Evil on a grand scale and a petty one (once using a terrorist attack just to ruin an enemy's retirement party), and with a bad habit of throwing his "friends" under the bus when it suits him, Kantor is out only for himself, and is as bad as even a Nazi can get.
Omega is the setting's resident Omnicidal Maniac. Once the heroic king of the first race of mortals, Omega realised that entropy was going to destroy the multiverse, and decided that he wanted to be on the winning side. Turning on Unus The Creator, Omega stole the Doom-Coil (the very device Unus had designed to fight against entropy) and reprogrammed it to spread entropy, feeding his own universe into it as fuel. In the process he defeated Unus, slew most of the other gods, and completely surrendered to his own hatred. Omega now travels universe to universe, corrupting and recruiting the greatest heroes of each 'verse into his army, before annihilating their home dimensions. He transformed the greatest physician of one universe into the sadistic Physician Friendly, promised to spare another hero's people only to turn them into mindless drones, and not only destroyed The Centurion's homeworld, but then tracked him to Earth-Prime, killing him and nearly wiping out Freedom City in the process. Concerned only with expanding his own dead realm (the so-called Terminus), Omega's goal in the extinction of all life across the multiverse.
Curse of the Crimson Throne: The followers of Zon-Kuthon, God of Envy, Pain, Darkness, and Loss tend to be a nasty bunch, with a penchant for torture and self-mutilation. Yet not a one of them—including quite possibly Zon-Kuthon himself—has ever come close to the level of atrocity perpetrated by Kazavon, Zon-Kuthon's one-time Champion. Essentially Vlad the Impaler in the form of a sixty foot Blue Dragon, Kazavon disguised himself as a human mercenary and offered to help the nation of Ustalav drive out the invading Orc hordes. Upon his victory, Kazavon set himself up as the dictator of the borderlands area, where he ruled with an iron fist, torturing to death all those who disagreed with him, including many of the soldiers who had served him faithfully up to that point. When his employer tried to rein him in, Kazavon flayed the man alive. He would go on to achieve truly special heights of depravity, holding torture parties, and orgies involving the undead, spreading his influence throughout the entire area, and having entire villages impaled for his entertainment. Kazavon was eventually killed by a party of heroes, but the madness didn't stop there. The Pure Evil of his soul contaminated his skeleton and threatened to resurrect him. The bones were crafted into seven Artifacts Of Doom and hidden throughout the country; contact with even one of them is enough to drive the wearer down a path of madness, murder, and ultimate self-destruction. When Queen Ileosa, Big Bad of the Adventure Path dons the crown, Kazavon influences her to murder her husband, turn Korvosa into a Police State, unleash a plague against the city's poorest citizens, and drain the lives of thousands in an effort to gain eternal youth (in reality, the spell will just resurrect Kazavon).
Second Darkness: Allevrah Azinrae was once an Elven cleric of Nethys, God of Magic, and a hero in the nation of Kyonin. That was before she, with the aid of the Demon Lord Abraxas, conceived of a plan to exterminate the drow. When the fellow members of her Government Conspiracy refused to go through with the plot, Allevrah murdered her critics, let her rage transform her into a drow, and fled to the drow capital of Zirnakayinn, where she murdered the matron of House Azinrae and took it over. Desiring revenge on Kyonin, Allevrah plans to drop a meteor on the capital; if successful this plan will wipe out Kyonin and trigger an ice age that will kill most life on the planet. After a failed attempt at using the city of Riddleport as a test target, Allevrah retreats to the Land of Black Blood below Kyonin, where she prepares to summon her meteor. She also allows her lover (who is terrified of her) to perform hideous experiments that reduce the victims to masses of screaming black liquid, cuts deals with an aboleth mind rapist and a neothelid, feeds prisoners to ropers, and orders her troglodyte henchman, Ornn, to eat a charda colony's children if they do not cooperate with her. Driven solely by hate, Allevrah shows just how far even the best person can fall.
Ezekyle Abaddon, known as The Despoiler, is the heir of the fallen Primarch Horus. Abaddon is consumed with hatred for all that lives, even his former master, and seeks nothing less than the extermination of all he can manage while uniting humanity under the dark banners of Chaos. As the only man with the support of all the Gods of Chaos, Abaddon has united the forces of Chaos under his banner no less than thirteen times in the Black Crusades. During these ventures, Abaddon storms the galaxy itself, burning and destroying all he can until the forces of the Imperium inevitably defeat him. Abaddon hardly minds as he is able to destroy and kill in great number, something he loves more than anything else. Willing to send countless numbers of his own to their doom and living only to destroy everything he can, Abaddon is the champion of Chaos and is one of the most evil and brutal beings in all of the dark world of Warhammer 40,000.
Fabius Bile is what you have when you cross a Mad Scientist with the dark powers of Chaos. Bile traverses the Galaxy more than any other member of the Traitor Legions for the purpose of experimenting whenever and wherever he can. On the world of Dimmamar, Bile changed the atomic composition of the air until the population was forced to ingest his transformative serums or slowly suffocate. He earned the name "Manflayer" in the Bray system for skinning those who opposed him to make a cloak of human skin, the survivors still alive to carry the garment behind him. Bile injects poisonous strands into every gene pool he can, with some experiments wiping out life in entire sectors. A sadist with a god complex and scientific curiosity, Bile is feared and hated by all who know his name and actions.
Vampire: The Masquerade: Sascha Vykos probably deserves a special mention here. A centuries old, genderless Tzimisce with a weird kind of god complex, he is described as making all the furniture in his mansion out of LIVING PEOPLE who are being kept alive through the power of Vicissitude, torture-raping people for shits and giggles and throwing them out when they go mad from too many disfigurements and generally being a massive dick (despite not having one). It's telling when most write-ups for his character quite frankly list his demeanor flat out as 'monster'. It's also no surprise that the rest of the Sabbat look to this nutter as a role-model.
In Hunter: The Vigil you can expect to deal with the very worst that the supernatural world can throw at you. Yet as the following slashers and cultists establish, sometimes you don't need to be a vampire or a werewolf to be truly heinous:
Larry Meeks, alias Captain Hook, is a small, pudgy, uninspiring man who runs a bait shop, dresses like he's going on a fishing expedition and has a fanatical hatred of anybody he thinks is better looking, more successful, or stronger then he is. A murderer since the age of fifteen, when he first determined to "prove himself", Larry baits in his victims with kindness, playing the role of the favourite neighbour or kindly uncle. Once he's won their confidence, he then subdues them with a gaffhook, and hangs them up in his fishing shack with thousands of tiny fish hooks embedded in their skin. Slowly draining his victims of blood over the course of several days, Larry then guts and cleans them "just like a pretty trout." Active for years, a recent invitation from the Subtle Collectors' Association (a cabal of like-minded killers) has convinced Larry that it is time to up his game, and he plans to move onto still bigger and better targets in the hopes of impressing upon his compatriots the is the best. Totally consumed by his need to hurt people, Larry spends most of his off time talking to his customers about his murders, using fishing metaphors to obscure what he's really going on about.
Harvey Ecks, known variously as The Rest Stop Killer, The Torso Maker, and (his preferred sobriquet) The Driver, had a dream when he was still in the womb. In it, he learned that by understanding the Dream Pattern, he would be able to gain total control over all reality. In his quest to achieve this goal, Harvey discovered that by forcing people to watch their limbs being amputated, he could make them reveal pieces of the Pattern. Once a roving killer who left headless torsos at rest stops, Harvey decided that this was too inefficent. Staking out a patch of the interstate highway, Harvey brainwashed diner waitresses, gas station attendents, and state troopers into blindly serving him, then installed video surveillance cameras on billboards. When he sights a likely victim, he forces his servants (who know him only as The Driver) to help him capture that person, whom he then tortures to death in the hopes that they will reveal more of the Pattern.
Thomas Salvatore, the head chef and owner of the Epicurean Club, is leader of the Pate de Fois Gras cult. A would-be chef, and gourmand, Salvatore was never able to get his restaurants to work. At least not until he'd visited Papua New Guinea and gained a taste for "long pig". Returning home, Salvatore recruited a gang of equally amoral chefs, purchased Briarwood Farms, and used it to raise his food of choice—human children. Keeping the kids locked in filthy cages, Salvatore and his compatriots force the children to bulk up on food and stimulants, before cutting out their livers and serving them to their patrons, none of whom have a clue what they are eating. Determined that he will be known as one of the world's greatest chefs, Salvatore plans to ride his new-found success all the way to the top—no matter how many children he has to butcher in the process.