Despite having no Sith Powers, Grand Moff Tarkin is just as horrible. In the only movienote except for a quick scene in Revenge of the Sith in which he appears, he orders the destruction of a heavily populated and influential planet purely to make a point to Leia, and seems quite willing to repeat the process ad infinitum if such is necessary to quash the Rebellion. The only emotion he shows at all at this prospect is a kind of cold satisfaction. The Star Wars Expanded Universe only makes him worse, by showing that he codified the Empire's policy of rule through fear as "The Tarkin Doctrine," and revealing that he first came to the Emperor's attention after he crushed a peaceful protest under his boots...by landing his Star Destroyer on them.
The Darth Bane trilogy by Drew Karpyshyn gives us these two:
Darth Bane is the one who both destroys and reinvents the Sith. Born the child of a simple miner, Bane became a soldier and eventually joined the Sith Academy at a young age. Soon enough, Bane became a devotee of the Dark Side and came to the conclusion the current Sith were weak and flawed. Bane would soon abandon anything resembling morality or compassion. When poisoned, he saved his life by slaughtering a father's three young sons in front of him, drinking in the man's agony before killing him as well. He received healing only when he threatened to torture a healer's young daughter to death. when he rejoined the Sith with the goal to purge them, Bane led them in a ritual that stripped a world of life and finally tricked the leader of the Sith brotherhood, Lord Kaan, into using a forbidden technique that wiped out the Sith and Jedi armies, leading their souls imprisoned in a horrible prison for the next thousand years.
Darth Zannah is Bane's first true apprentice who submits herself to the Dark Side as a young girl after she crosses the Despair Event Horizon. Proving herself to have the potential for evil, she murdered the crew of a small ship, including children, to secure passage to rejoin Bane. After ten years with Bane, any kindness or innocence Zannah once had is dead and gone. She has no compunction betraying her lover to his death, never having cared about him to begin with. When she discovers a terrorist in the same conspiracy was her lover's other woman, Zannah reveals her talent as a Mind Rape specialist: she uses her sith sorcery to destroy the woman's mind with horrible visions that make her claw out her own eyes until her mind is destroyed, save for a small pocket of consciousness where she will suffer until death. After a duel with the Jedi, Bane is critically injured. Unwilling to let him die before she has all his teachings, Zannah tricks her cousin and the same healer Bane blackmailed years ago, before she slowly hacks the healer to death with her lightsaber and uses her powers to drive her cousin into insanity so the Jedi will kill him, thinking he's the Sith Lord. While Bane is ice cold, devoid of emotion and feeling, Zannah is violent and openly cruel and sadistic, preferring to cause pain mentally. She is a worthy heir to the millennia long evil of the Sith.
Lord Cronal, also known as Lord Shadowspawn and "Blackhole", was the Big Bad of Luke Skywalker and the Shadows of Mindor. Cronal grew up worshiping the Way of the Dark, an ideology which focused on nihilism and instilled in him the fanatical belief that the only true power was the power to destroy. Seeing the "creation" of his own daughter, Sariss, as a transgression against nihilism, he disavowed her and gave her to the Prophets of the Dark Side to be raped,even participating himself. Though Cronal initially served Emperor Palpatine, in reality he held contempt for the Emperor and plotted to overthrow him because Cronal viewed him as not destructive and evil enough for his tastes. Following Palpatine's death, Cronal declared himself the New Galactic Emperor and waged war on the New Republic. Cronal's Imperial splinter faction embarked on a campaign of wholesale slaughter against multiple worlds, conquering the Inner Rim in the process. In order to act as his puppets and decoys, Cronal kidnapped, lobotomized and mind-controlled multiple Force-sensitives to act as extensions of himself whom he could bodyjack should his own frail, elderly body fail him. His plan during the Battle of Mindoir being to take over the body of either Luke or Leia both due to the siblings' force sensitivity and to prolong his own life. Cronal also had kill-switches implanted into the entirety of his forces, and, when his attempt to switch to a Skywalker body failed, he sent them all on suicide runs against the Republic while he escaped. This necessitated the death of every Imperial troop under Cronal's command, 50,000 people total. In the end, Cronal was a man so completely nihilistic, that his ultimate goal as Emperor was to create a never-ending cycle of entropy until all of existence, including himself, was destroyed.
Another one is Onimi from the New Jedi Order. This is the dude who masterminded the most devastating war in Galactic History, who indirectly caused the deaths of trillions and the extinction of whole species, just so that he could become a god. He even tells Jaina that he's looking forward to "exterminating everyone and everything in this foul galaxy." During his Motive Rant to a captive Jaina, he does try to cast himself as a Woobie, Destroyer of Worlds, but she (rightly) just thinks he's a lunatic and is repulsed. It's made clear to the reader and both Solo twins that, by the time of the Final Battle, Onimi's motivation is For the Evulzat best. Even worse, it's also strongly implied that his plan to descend the Galaxy into darkness had actually succeeded.
Ysanne Isard, Director of Imperial Intelligence and Emperor in all but name during the X-Wing Series. Along with the traditional things that come from being at the head of the Empire, like oppressing nonhumans and ordering strikes against the Rebels, and what she did to the previous people in power, she ordered the creation of the Krytos Plague, which causes infected nonhumans to die horribly, and set it loose on Coruscant before the Rebels captured it. This so that the New Republic would both bankrupt itself trying to treat the infected and tear itself apart along species lines, as resentment towards immune humans rose. It might not have gotten that bad, but that was the intent. And then there's how she left Coruscant. And Lusankya. At one point, Wedge thinks to himself that Isard might not have been evil on the scale of the Emperor or Darth Vader - aside from Luke and sometimes Leia, few people in that 'verse consider Vader anything but evil even after his Redemption Equals Death - but that was small comfort to the millions who died at her hand.
Moruth Doole. He keeps a harem of Rybet females he rapes, and he enslaves the children, making them package up the spice mined. Then, when they're old enough to challenge him, he kills them. He runs a massive slaving operation with high fatality rates, without a glimpse of remorse or humanity. He's also fond of feeding any enemy slaves to the giant spiders that inhabit the spice mines
Doctor Evazanmurdered one of Zak's friends because he'd found that fresher corpses made zombies who were smarter and more useful. He then arranged for the resulting Undead Child to bring Zak to him and injected him with the zombie serum and a poison that would make him go into a coma so deep, he'd be mistaken for dead and Buried Alive.
The Big Bad of the first six books, Borborygmus Gog. All of the threats in those books are because of him and his suboordinates. He had a planet become alive and hungry, and deliberately stocked it with a species that would make people feel welcome so they could be eaten, and then left it out to be discovered. He hired Doctor Evazam. He injected Tash with The Virus while on the way to try and kill her uncle. He tested the Nightmare Machine on random young amusement park visitors. He spent months posting stories about Jedi to the Holonet, which is so censored by the Empire that his is virtually the only source, to draw in and befriend people interested in Jedi, with the intent of making them agree to meet him so he can have their Life Energy siphoned - all in the hopes that one of them will be Force Sensitive so he can study it. And then there's Kiva.
Montross from Bounty Hunter. His first introduction shows him coldly gunning down a helpless bounty who's wanted dead or alive, simply for the pleasure of murder. As the man desperately pleads, his bounty is worth more alive. Montross replies "You're worth enough dead." Later on, while trying to beat Fett to the big prize that they're both after, he tortures Fett's non-combatant friend and mother-figure to find out the location, mortally wounding her in the process. Then he sets her entire space station to explode, hoping to kill Fett. He fails, but undoubtedly slaughters countless civilians who were on the station. Fortunately, he gets a karmic death when Jango leaves him to be ripped apart by the Banda Gora.
Lieutenant Bella and Cavik Toth from Jedi Starfighter. They drop bioweapons on a town filled with civillians, and when the test is successful Bella still tries to level the city purely because she enjoys, failing only because Reti and Adi intervene. She also violently threatens her subordinates. Toth, meanwhile, kills his aids with the THX weapon simply because production is behind schedule, and develops weapons to use against the clones, testing them against innocents. Nym's aid describes him as a ruthless monster.
From Knights of the Old Republic II, Darth Nihilus is an Eldritch Abomination who drains the force energy of entire planets to sustain himself, and while that might seem somewhat justifiable considering he has to do it in order to survive, he also augments it with pure sadism: after obliterating the Jedi gathering on Katar and taking a good deal of the Miraluka with them, he finds Visas Marr and makes her his apprentice- mutilating her face in a cruel mimicry of eye sockets, and mind raping her into submission to seal the deal.
The Emperor is a pointlessly cruel man, murdering his own parents as a child, torturing his mother for months first. He also drained the energy from a planet, forced Revan and the Exile into his mind to torture them, telepathically tortures Exal... it says a lot when the ancient Sith Lords preferred Exar Kun over this guy. It says even more when the Dark Council plotted to overthrow him. In more recent times Kira endured even more thanks to him. She fled the Sith because of his evil and during the Jedi Knight's battle with Angral, he possesses Kira purely to do away with her friend. Why? Because he saw visions that her friend might be the one who will defeat him and being Dangerously Genre Savvy he's decided to take the hero out now while he or she is still weak.
When the same game's Darth Jadus is first introduced, he appears to only differ from his fellow members of the Dark Council in that he is enigmatic. It gradually becomes apparent that Jadus is twisted even by the brutal standards of the Sith. Jadus fakes his own death at the hands of a terrorist so he can secretly kidnap several hundred people and torture them into becoming his mindless servants. He then gives incredibly powerful Kill Sats to terrorists so they can use them to kill countless Imperial citizens, all so he can come back from the dead and save the day, increasing his political power. To prevent the other members of the Dark Council from figuring out his plans, Jadus has his insane daughter, Darth Zhorrid, succeed him on the Council as a distraction. If Zhorrid somehow manages to survive Sith politics, Jadus plans to have her killed. Jadus plans to use the power he will gain from his plans to repeat the tortures he inflicted upon the people he kidnapped with the entire Empire.
Chantique the slaver is a leader of the slaving operation Crucible and as the Magister Impressor is responsible for educating and teaching new slaves. Chantique proudly presides over a legacy of misery and torment, forcing slaves to battle one another in pointless gladiator fights, with one slave in particular having to carry the memories of every other slave who had ever suffered in the past. After the slave's suicide, Chantique proceeded to mock his would-be savior on the meaninglessness of his death. A brutal sadist who was not above outright murdering her vicitms, Chantique also possessed a grudge against her old rival Jarael and stopped at nothing to make her suffer and destroy all the relationships she had formed. The crowning moment of cruelty for Chantique was seeing that ever since she rose to lead Crucible, she celebrated by having one of her old schoolmates buried alive. When she learned her father was coming to collect her old classmates, Chantique buried them all and gleefully mocked him that he should have taught them how to hold their breath.
Demagol, born as Antos Wyrick. Demagol became one of the most feared and loathed beings in Mandalorian history. Known as 'The Flesh carver', Demagol hungered to learn the secrets behind the Force by torturing the Jedi who fell into his grasp, even the children when children are honored and protected by Mandalorians. The experiments would often end in excruciating vivisection. Revealed to have once sold his own daughter into slavery at Crucible, when faced by her after stabbing her in the back, Demagol snarled he only wanted "The good ones. The ones that worked!. Combining a complete lack of empathy with sadistic scientific curiosity, Demagol stands as a unique monster in the Star Wars franchise and centuries after his death, his name had become a curse to the Mandalorians.
Haazen began his career in evil as a failed, jealous Padawan who murdered his former best friend Barristan Draay during the Great Sith War. His body rebuilt and scarred, Haazen's soul became as twisted as his outward appearance. Ruthlessly manipulating other Jedi into murdering their own Padawans, Haazen had Coruscant bombed from orbit to initiate a war that would allow him to destroy and rebuild the Sith and Jedi orders while ruling them from within. Haazen kept the true leader of the Convenant and the only one who might have stopped him in a state of horrible suspended animation where she could perceive every atrocity he manipulated her son in to committing, including the deaths of children. A man defined by jealousy and hatred, Haazen's grand goals of plunging the galaxy and causing the deaths of countless innocents while perverting the Jedi Order towards darkness were nothing more than an attempt to feed his ego and spit on the grave of Barristan Draay.
A minor Star Wars character who only physically appears in one comic of the "Dark Times" series (which depict the aftermath of Episode 3, when Palpatine has all but total power in the galaxy) manages in his brief screentime to become a "shining" example of the trope as well: Dezono Qua. He even has a Meaningful Name tipping off readers knowledgeable in Native-American myths about his nature. Qua is a minor human noble who dabbles in slavery, and is the purchaser of one hero's young daughter. When he's confronted, Qua eveals the reason he deals exclusively in child slaves: he eats them. When asked why he has children purchased solely to kill and cook them, Qua's response? Because I could. Even Jedi Master Dass Jennir was so appalled at his horrific crimes and callous attitude that he decided to just shoot him dead.
Few monsters have thousands of years to perfect the art of killing. Durge is the exception. a member of a normally peaceful species, Durge distinguishes himself from the normally docile Gen'dai race by being a brutal psychopath who enjoys the agony of his victims. Throughout the Clone Wars, Durge was one of Count Dooku's most feared commanders. plunging headlong into battle to kill as many opponents as he could, with a special emphasis on civilians, clones and Jedi. Through portrayed as a Generic Doomsday Villain in the original Clone Wars 2003 TV series his appearence in the comics makes him much worse. When we were first introduced to Durge, he was seeing testing poison gas bioweapons on a civilian Gungan colony on Naboo's moon, leaving no survivors. Durge proceeded to gloat that with all the gungans he had already murdered, the Jedi he would get to kill presently, and the fact that he was going to use the gas on Naboo's cities the following day? That made it a damn good week for him.
Beginning his career as an arrogant, brash Jedi Apprentice who considered aliens to be inferior to humans, Exar Kun found himself drawn to the Dark Side. Under the tutelage of the ghost of dead Sith Lord Freedon Nadd, Kun plunged down a dark road where he embraced the darkness and destroyed his master's spirit, claiming lordship of the Sith. As the Dark Lord, Kun would enact alchemical experiments upon his followers, a species called the Massassi, twisting their bodies into weapons of war for his use. Kun would corrupt other Jedi apprentices by having them possessed by Sith Ghosts and sent them to murder their own masters. In the Sith War, Kun marched into the very heart of the Republic, murdered the Supreme Chancellor and delivered a grand speech to the Republic before killing his own Jedi Master in a duel. Kun would then trick several followers into initiating a supernova that would devastate entire star systems, including a genuinely loyal servant, mostly because the man's undying devotion irritated him. When cornered at his home base, Kun sucked the life from the entire Massassi race to preserve his own existence and trapped himself within his temple for four thousand years. When his spirit returned to haunt Luke Skywalker's students, he proved himself as cruel as ever by manipulating Luke's prized students, burning one alive from the inside out and having another use a superweapon to destroy solar systems.
Vul Isen is a scientist whose job is genocide. when Darth Krayt decides the Mon Calimari species needs to die, Isen is appointed as the one to expedite the process. Isen's first plan is to create a sea monster known as a Leviathan, that steals the souls of its victims and imprisons them in eternal anguish. When this is destroyed, Isen develops a toxin to kill all life on the planet. Despite the best efforts of the galaxy, Isen exterminates 80 percent of life on Mon Calimari with nothing more than satisfaction at his own brilliance. He then proceeds to attempt to exterminate all life on another planet that defies the Sith after having refugees hunted down
The Emperor of the One Sith, Darth Krayt was a great Jedi Knight once, but that time is long past. Seizing control of the Imperial throne, Krayt establishes a military dictatorship that violently destroys any who might be a threat. When a single Mon Calimari shipman is captured on board a rebel cruiser, Krayt flips out and kills him after ordering a summary execution of ten percent of the Mon Calimari species while the rest will be forced into camps and exterminated later. Suffering a betrayal from his most trusted follower, Krayt later returns from the death, now of the belief the galaxy must experience death as he had before launching a violent, galaxy wide campaign, with the plan to manipulate his enemies to use a toxin against Coruscant, the most populated planet in the galaxy, that will kill everything on the world. Except the Sith.
Jango Fett's archnemesis, Tor Vizsla, founder of the Death Watch extremists bent on returning the Mandalorians to their worst warmongering ways and the reason for every tragedy in his life. When Jango was a young boy, his entire family was killed by Vizsla and his men when they got caught in the crossfire of his war with Jaster Mereel and the True Mandalorian faction. Later on, when Fett had succeeded Mereel as leader of the True Mandalorians, Vizsla conspired to trick the Jedi into believing that the True Mandalorians were slaughtering political dissidents (the "dissidents" were actually armed insurgents) along with women and children. The evidence for the latter was provided by Vizsla's men. The result? A horrific battle between the Mandalorians and a task force of Jedi that left almost everyone on both sides dead.
Star Wars: Knight Errant brings us Lord Odion. Born as the heir of the mighty Sith Calimondra family, Odion had an unusual affliction where the existence of others caused him physical anguish. Finding a Sith solution, Odion found joy in the murder of others to the point where the murder of sentient beings became a high to him. Having learned to twist the minds of others, Odion found joy in forcing helpless captives to attack carnivorous monsters and restored their minds to savor their agony when they were devoured. Leading countless campaigns that claimed the lives of innocents in his battle with his hated younger brother Daiman, Odion found a solution to the problem of existence: an ancient Sith helmet. An artifact that would initiate homicidal and suicidal insanity in living things. To power it up for its task, Odion had imprisoned every child in his realm to deprive them of hope and used their terror and anguish to power his helm, being but moments away from ending all life before being stopped. Odion's final goal was to exterminate all life and rule the graveyard he'd made of the Galaxy.
Pong Krell. When introduced, he seemed to be the standard General Ripper, just following orders no matter what the casualties. This is unexpected from a Jedi, but there have been some like that in the past (like how the Jedi Exile and other Jedi were in Mandalorian Wars, or in the last Sith War a thousand years prior) and thus seems like a case of Good Is Not Nice. Then he orders the death of Jesse and Fives, as well as a court martial for disobeying orders after they saved the day by doing so. Then he truly crosses the Moral Event Horizon-he plays two armies of Clone Troopers against each other by saying that there were infiltrators, having them kill each other. And shortly after that, he confesses that he's been trying to lose as many clones as possible because his plan was to sabotage the Republic war effort from within in order to ingratiate himself to Count Dooku and become the Sith Lord's next apprentice.
Keeper Agruss the Zygerrian is a slaver, which is bad in itself, but goes above and beyond the call of duty on that. In particular, he has no problem casually murdering dozens of slaves if it means breaking a Jedi, tortures and kills other slaves for Obi-Wan's defiance, not only making his help unproductive but actually alienating those he seeks to protect. He truly earns his credentials at the end. The Republic has arrived and it's quite clear he's lost. What does he do? Set the system to dump the slaves into the abyss, smashes the panel to prevent Obi-Wan from saving them (Ahsoka manages to get them to safety), then gloats about how Obi-Wan can't retaliate because he's a Jedi. His death at Rex's hands was very deserved. Even the queen had a redeeming moment at the end. This guy was just straight-up evil.