Monster / Sonic the Hedgehog
Dark Enerjak. Notice what's behind him.

Robotnik: "Snively, what color is my heart?"
Snively: "I don't see a heart, sir..."
Robotnik: "Exactly."

Sonic the Hedgehog, though usually a lighthearted franchise, becomes much darker when these characters show up.
Video Games

Comic Books
  • Sonic the Comic: Dr. Zachary, Knuckles's Arch-Enemy, is introduced as a seemingly harmless old man and one of the last of the echidnas. Zachary tricked Knuckles into leading him into the Emerald Chamber, whereupon he shattered the Master Emerald and used the Chaos Emeralds to empower a reprogrammed machine of his. Zachary gleefully allowed the inhabited floating island to fall onto Mobius as a "monument to his genius", intending to butcher the population of the planet Mobius for his own amusement before seeking revenge on Knuckles's people. In his second appearance, rebuilt as a cyborg, Zachary leads Robotnik to the location of the floating island's villagers so Robotnik could use them as components in a biological computer, which would consequentially kill them, a process which Zachary happily watched over. Possessed of a vindictive streak matched by few, Zachary manages to outshine most of the comic's other antagonists despite his relatively minor running as a villain.
  • Archie Comics' Sonic the Hedgehog:
    • The original Dr. Ivo Robotnik, during the Great War, intended to test weapons he'd developed for the government on his own people and fled to the Mobians' side when he was to be punished for his actions. While acting as warlord for King Acorn, he secretly captured a peaceful village of Mobian monkeys and utilized them in an early attempt to create mechanical slaves by surgically implanting cybernetics, and of that entire village only one survived the process. Afterwards, he sabotaged the roboticizer developed by Charles Hedgehog just as it was to be used on his wounded brother Jules, making him think he'd turned his own brother into a mindless drone. When Jules's wife discovered the truth, Robotnik had Jules toss her into the Roboticizer and transformed into a robian as well, orphaning Sonic and crushing Charles's spirit to the point of retiring from being a Science Minister to running a chili dog stand. He then did similarly to Tails's father Amadeus on the day of his birth, making it seem as though Amadeus could care less for his newborn son and subsequently shattering his wife's heart, throughout which Robotnik maintained a straight face. Afterwards followed his betrayal of King Acorn and the takeover of Mobius, during which countless families were broken and millions enslaved, and over the course of a decade, he'd wreak havoc upon planet Mobius, destroying the ecology and pushing more than a few mobian breeds to the brink of extinction. And just prior to his death, he displayed a deeply disturbing relish at the thought of murdering one of his own kind and was positively gleeful when he got the chance to use his latest and greatest weapon on a fellow overlander.
    • General Kage Von Stryker is the leader of the Eggman-backed Dingo Regime. After ousting his father Helmut from power, Kage led his people in conquering Angel Island, imprisoning the Echidnas in what were clearly concentration camps, which led to countless deaths. He personally tortured Knuckles's father for the location of the Master Emerald, tried to wipe out the last echidna village on two occasions, and during the second attack boasted that he had crushed more Echidnas under his boot than he could remember. While like Helmut, Kage also hated the echidnas, unlike his father, Kage just wanted to Kill 'em All.
    • Dr. Finitevus was once a fellow scientist of a race of prosperous echidnas, until a freak accident changed his looks and intelligence. Despite his newfound genius, he was declared mentally unstable and was to be put down. He escaped and later sold out his home to Dr. Robotnik, resulting in everyone who had once lived there dying later. In addition, Finitevus manipulated four whole sides in a war—Dingoes, Echidnas, and two Dark Legion factions—into providing the fuel for the fire of his master plan, which is him deciding the world is hopelessly corrupt and wanting to purify fire. He, to this end, manipulated Dimitri, a head in a jar, into providing secrets and information, and then manipulated Knux into becoming Knuxerjak and starting a battle with his former friends after the whole corruption from power thing. Finitevus's expression at the scene where Knux becomes Enerjak shows he is not only mad, but completely delighted, even as his betrayed allies watch on and denounce him.
    • Hunter, as his name would indicate, would hunt down certain specimens of Mobian, and he debuted trying to do just this to Knuckles. Hunter revealed that not only did he hunt down Mobians, but that he would also claim their heads as trophies in the same manner of a real life hunter. After being defeated he would go on to work for Eggman and would become a gleeful participant in the genocide of the Echidna and other Angel Island inhabitants, capturing droves of poor souls to be sent to the Egg Grape Chambers, where they would perish in a slow, horrible fashion by having the very life drained of them. Though only a minor villain in the long run, his actions put him well beyond the point of redemption and into the realm of the truly malevolent, despite some pretensions earlier about having nobler characteristics.
    • Dark Enerjak, the tyrannical ruler of another dimension, is actually a version of Knuckles the Echidna, fueled by a desire to make the world a better place, whose Chaos Powers went out of control. While he started out noble, he gradually went more and more insane as time went on, before he considered everyone an enemy. Among his atrocities are the sinking of a continent; the destruction of several enemy cities, such as Station Square, as punishment for their defiance; and the capture of his strongest opponents and ripping their souls out—victims include Sonic, Shadow, Tails, Amy, Sally, and all the villains. He turns the souls into robotic slaves known as "Prelates". His soulmate was one of his victims, and he made sure not to use her soul to form a Prelate until the final battle, when he summoned her to fight their daughter, much to said daughter's disbelief. Dark Enerjak's mainly motivated by his own amusement, despite saying how much of a hero he is at other times. He allowed the Freedom Fighters to exist to give him something to do, sent soldiers to other dimensions to try and find new worlds to conquer, and continued to indulge Silver because of his power to actually give the living God a challenge. He derives joy from people's suffering, as seen with how he forces his own daughter to fight her mother and his former mate and how, upon seeing how upset Silver is at Enerjak using the souls to make his Prelate Army, he summons them all at once.

Sonic SatAM
  • Dr. Julian Robotnik is the absolute ruler of a roboticized Mobius, and is a far more chilling incarnation of the normally-bumbling character of Robotnik. A ruthless, ambitious backstabber with a series of treacherous moves to his count in the name of self-interest, Robotnik's first betrayal was his sealing of the wizard Naugus in the Void. Later becoming known to the King of Mobius, Robotnik became instrumental in the King's own military victories before deciding to take over Mobius himself. Staging a coup d'état and overthrowing the King from Mobotropolis, Robotnik stole the technologies of the Roboticizer from Charles Hedgehog, and used it to forcefully convert almost all of Mobius into a polluted dystopia. Reducing all of Mobius's inhabitants to his robotic slaveswhich they morally object to and fight against—Robotnik made fierce enemies with Charles's nephew Sonic and the rest of the Freedom Fighters. Robotnik would demonstrate his psychopathic nature again and again, attempting to kill, torture, or otherwise roboticize the Freedom Fighters, happily willing to jeopardize the ecosystem in doing so. Robotnik also coldly abuses his own minions, his diminutive, weak-willed nephew Snively among them, and condemns them all to die for the sake of his own life in the finale. An emotion-deficient fear-monger with nary a comedic quirk to his name, Robotnik proved one of the most vicious incarnations of Robotnik seen in the franchise.