The Book of Vile Darkness gives us The Dread Emperor. An imposing, 2 meter tall man in golden armor, the Dread Emperor keeps children chained to his armor at all times with any damage done to the Emperor himself transferred to the children. The Emperor uses these children as a bait for any heroes foolish enough to take him on and will often destroy entire city blocks when he's a mind to. When a hero challenges him, the Dread Emperor feels no compunction slaying hundreds simply to kill a target or mentally enslaving any civilians nearby and using them to attack his opponent for him.
In the Mystara supplement:
Baron Ludwig von Hendriks, The Black Eagle, an insane aristocrat who rules the Barony of the Black Eagle. von Hendriks has raids conducted where innocent halflings are often taken and enslaved and is most infamous for the tortures he inflicts. In the Black Eagle Barony, being accused of a crime gets you before his court. The problem is von Hendriks only cares how loud you can scream and devises nasty tortures that he's used on countless innocents.
The Baron's former right hand man Bargle the Infamous is just as evil. As the Black Eagle Barony's number 2, Bargle enforced its brutal reign of terror, and deserted it when it fell. Bargle had used many innocents and captives in twisted experiments. he later participated in destroying the flying city of the gnomes and captured a well known cleric to father his child on her, after presenting her with the tortured corpse of her husband.
In the 3.0 adventure The Bastion Of Broken Souls, we have Ashardalon, an incredibly ancient, powerful and wicked red dragon. Already a cruel and rapacious creature in his own right, after having been mortally injured by a powerful druidess who didn't live long enough to celebrate the deed, he managed to sustain himself by substituting his own failing heart with a denizen of the Abyss - a Balor, no less! Of course, the demon was not enthusiastic about this, and Ashardalon had to search for another way to extend his lifespan... hitting the jackpot when he discovered an Eldritch Location deep inside the Plane of Positive Energy where the souls of every living creature are born and reside before getting a body. There, he settled down with a few of his servants and began devouring pre-incarnated souls in order to live forever. This act is causing countless being across the universe to be born without a soul, horribly alive and at the same utterly dead... but if the PCs call the dragon out on his crimes, his answer is that he doesn't care if he fucks up the multiverse, as long as he gets to enjoy eternity as the self-styled "ultimate predator".
Ashardalon remains just as big a prick in 4th edition, only the whole Font of Souls business is slightly greyer.
Borys 'Butcher' of Ebe: a former member of the Champions of Rajaat, a cult of genocidal lunatics who attempted the complete extermination of all non-human sentient races (and SUCCEEDED with many of them) by using the environment-raping defiling magic and turning the world of Athas into a warped desert dying world... this before going Starscream on the Champions and their master, absorbing them (and killing an entire city in the process) and turning into a draconic Eldritch Abomination.
Takhisis, the head of the Pantheon of evil deities. Unlike her fellow evil gods who at least have some redeeming straight to balance them out (her husband, Sargonnas genuinely cares for his Minotaurs and her daughter Zeboim loves her son), Takhisis cares only for her lusts, greed and power. After mothering the second generation of Chromatic dragons, Takhisis brutally killed two of them solely to make a point to the others. Takhisis threatened the world multiple times, ending her armies out to commit a host of atrocities with her servants enslaving whole countries in living nightmares. Takhisis betrayed all her gods by stealing the world of Krynn at the conclusion of the Chaos Wars against Father Chaos after she abandoned her followers and forced every soul of those who died into an army of the dead.
Maladar the Faceless, the cruel emperor of Taladas. When the series begins, Maladar has been dead for centuries and is remembered in terrifying legends. Flashbacks reveal Malador was The Caligula of the old empire who conducted ethnic cleansing, invented torturous deaths for thousands (he was particularly fond of impalement) and kept a slave boy he molested until the boy managed to poison him. After death, Maladar's influence lived on, as his soul was trapped into a small statue of his likeness. After he's revived he spreads his influence to modern Taladas, inciting brutal wars to create enough bloodshed to prepare his resurrection. His initial choice of vessel is an innocent boy, but he simply bodyjacks the boy's father instead and enslaves goblin races to send them to their deaths as a cover for him to awaken his army and conquer Taladas anew.
Malystryx, one of the first and mightiest dragon overlords is a titanic and brutal monster who carved out territory by simply incinerating anything in her way. After settling in Krynn, Malystryx hunted down and murdered other dragons, taking their skulls and inciting a dragon purge . Using the skulls as a sinister totem, she mutated the land around her into a hellish wasteland and allied with barbarians and ogres to destroy and enslave any innocents left. Malystryx killed her own mate to complete her totem and proceeded with a genocide of the Plucky Comic Relief race, the kender, only prevented by the Heroic Sacrifice of the warrior Riverwind. Establishing herself as a brutal tyrant, Malys would occasionally attack random targets for her own amusement, delighting in the instinctive fear she spread. Without doubt, Malys was the most evil dragon ever to set foot in Krynn.
Myrkul, the old god of death before Kelemvor. Unlike Kelemvor, who has a firm set of morals and only tolerates the Wall of the Faithless because it's necessary for the continued existence of the other gods, Myrkul was an utter sadist who took gleeful joy in witnessing the agony and suffering the Wall caused. He is stated to have ruled his faith with an iron fist, purposefully leading his followers through sheer fear of his retribution, and executing anyone who defied him, be they man, woman or child, in sadistically brutal fashions, such as burning them alive in a giant furnace. His greatest atrocity, however, was what he did to one of his best priests, who had had enough of the injustice of the Wall and attempted to destroy it. Initially, he just left the priest to rot on the Wall, but just before the priest fully merged with the Wall and attained some measure of peace, Myrkul decides that he hasn't suffered enough, and transforms the priest into a mindless being of pure, unbridled hunger that feeds on souls, then sets it loose upon the world, knowing full well the chaos it will surely cause.
Cyric. In the aftermath of the Time of Troubles, when the three prime gods of evil died, Cyric was raised to godhood for his evil in order to replace them. Cyric proceeded to prove his cruelty by murdering and oppressing other gods, sending his servants to steal faithful worshipers from the afterlife to torture in his realm, instituting cruel punishments on the dead. Cyric invented new ways to drive beings made, even succumbing to them himself for a time. Even after regaining his sanity, Cyric gleefully murdered his enemy Mystra, the Goddess of Magic, inflicting the hideous Spellplague that scarred the face of Faerun forever and claimed many lives.
Lolth is the goddess of the Drow, as well as the goddess of spiders and chaos. Once a member of the Elven pantheon, the Seldarine, and named Araunshee, Lolth betrayed the Seldarine out of growing malice, discontent, and lust for power. She betrayed her husband, the chief elven God Corellon Larethian to his enemy, the Orc God Gruumsh and tried to help the destruction of the Seldarine. When she failed, Lolth fled to the Demonweb Pits and led her chosen race, the Drow, into the Underdark, becoming a twisted and cruel goddess. Lolth would soon trick her own grandson into absorbing the essence of a demon lord to corrupt him into serving Lolth as her champion forever. Lolth had the Drow establish a society absolutely built on sadism and backstabbing, solely for her own pleasure, with cruel edicts consisting of demands for the sacrifice of every newborn male child at birth and demands for her priestesses to carve out the hearts of males they grow too fond of. What sets Lolth apart from other evil Gods is that she cares nothing for her race or her worshipers, viewing them as merely tools for power. For her own amusement, she will capriciously withhold her favor from loyal Drow just for the pleasure of seeing them die. Lolth has nothing less than genocidal fury towards all surface elves and routinely directs her followers to murder them. After becoming more powerful, Lolth proceeded to purge the Drow pantheon of other deities, ending her victory by destroying her own daughter, Eilistraee with savage glee.
Rivalen Tanthul, one of the 12 Princes of Shade, the remnant of the empire of Netheril. Rivalen is the most powerful of his brothers and while his father and siblings are certainly ruthless, they genuinely love one another. Rivalen is the Nightseer, or high priest of the Dark Goddess Shar and murdered his own mother to instill his family with the loss needed to turn them to Shar's worship. Rivalen mentally tortures his younger brother Brennus with the vision of the murder and even mocks his father, High Prince Telamont, about loving Rivalen too much to stop his mad plans. In the vision, Rivalen's dying mother simply asked Rivalen to hold her hand so she wouldn't be alone, to which Rivalen calmly denied her request to savor her suffering. Rivalen engineers massive casualties and Shade's takeover of the lands of Sembia, savoring the bitterness and pain of those he betrays. A century after Sembia's takeover, Rivalen attempts to finish Shar's manifestation, revealing his ultimate goal: to die and take all that exists in Toril with him. Defined by his cruel nihilism and utter lack of regard for any living being, Rivalen sought deaths on scales that not even Gods such as Cyric would attempt.
Iuz The Evil, the cambion child of the demon lord Graz'zt and the human witch Iggwilv. In his early career, Iuz rallied an army and launched a series of brutal campaigns that gave him the name 'Lord Of Pain' or the atrocities he committed on innocents. Eventually, Iuz gathered powerful sorcerers and drained them of life to add their powers to his in a path to godhood. Eventually becoming a demigod, Iuz ended up feared by all those who knew him, with no care towards either of his parents or any in his way. He eventually resorted to murdering his formerly loyal servants and replacing them with Fiends solely for convenience and power.
Carman Skiric in Battle Machines. He 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. Arguably a fine politician when first presented, 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.
In 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. He's the source of basically everything wrong with the 2056 juncture that isn't his ostensible boss' 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.
Magic: The Gathering brings us Yawgmoth, who 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. A small, pudgy, uninspiring man, Larry 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, and 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.