Go Nagai has written many stories in his long career, and they have many horrible villains.
Jinmen, the depraved turtle-like demon eats humans alive to devour their souls, leaving an imprint of them on his shell while they are trapped in agony. Jinmen targets vulnerable humans on purpose, with children being a favorite meal, including a 10 year old named Sachiko who had befriended Akira. Those he devours are left alive and conscious, but trapped until Akira kills him.
Candy from the AMON side story was a Serial Killer before achieving great power. In the post apocalyptic world, Candy ekes out a power base by convincing humans there is no such thing as Devilmen, resulting in purges of countless peaceful humans believed to be demons. Candy takes delight in the deaths of the innocent and simply enjoys wielding power over others.
From the spinoff Violence Jack we have The Slum King, the Big Bad of the story. The most powerful warlord of the Kanto region, the Slum King maintains his power by brutally murdering any in his way and violent conquest where nobody is spared if they resist. The Slum King allows rape, murder and looting and when someone really annoys him, he'll simply subject them to Cold-Blooded Torture. Perhaps the worst thing he does is the 'Dog-People': cutting off all the limbs of a victim and making them crawl on peg-leg stubs while he keeps them as a Sex Slave.
Lan Asuka from the anime of Devilman Lady is revealed as the Big Bad of the series and to have been manipulating the heroine Jun the whole time. First awakening Jun's powers as a monstrous Devil Beast by letting another attack and try to rape and eat Jun, Asuka formed a lust towards Jun's alter-ego of Devilman Lady and constantly toyed with her and her friends. Asuka manipulated conflicts throughout the series between humans and the mostly peaceful Devil Beasts, resulting in camps formed to hold and exterminate all the Devil Beasts. Asuka lured Jun to her and revealed her true intent: despising the world and all in it, Asuka had been born both male and female. Her ultimate goal was to use Jun to become a new God of the world. Asuke paralyzed Jun and then, demanding Jun 'prove' her love for Asuka, raped a traumatized and terrified Jun before informing her Jun had been born solely for her and would now die for her before disposing of her. When Jun returned, the newly deified Asuka mocked her, saying being perfect was too good for a 'hideous creature' like Jun before trying to destroy her in their final battle. Valuing her own ego above all else, Asuka was a manipulative and cruel embodiment of lust, cruelty and selfishness.
For Baron Ashura, "good" and "evil" are meaningless words made up by human beings. He/she is perfectly capable of hitting children (once also having blown apart the wheelchair of a disabled girl to prevent her from escaping), murdering little dog cubs or commiting worse atrocities if he thinks it will further his/her goals, like machine gun several survivors of a shipwreck as he/she gloats they will be revived like cyborgs and turned into his/her slaves or like slaying a hostage or pawn after he or she has outlived his or her usefulness. He/she is easily Hell's most brutal henchmen, usually the most loyal, and would do anything to turn Hell's dreams into reality. Under his/her command, his/her Mechanical Beasts set whole cities on fire, bombard them with missiles into smoking ruins, cause earthquakes to bring them down, cut ships in halves, murdering the whole passage, shoot or blast whole planes down, blow up trains and buses and thus kill thounsands of people.
Before Hell met him and saved his life, Count Brocken was a Nazi officer who was considered particularly blood-thirsty by his own partners. He also is the first of Hell's henchmen to kill someone on panel on the manga even before Ashura, and he is capable of joking as he cuts heads off.
In one of the alternate realities from Shin Mazinger Zero, Dr. Kabuto himself. He murdered his son Kenzo, tried to murder his grandson Kouji, and raped and murdered Sayaka in front of an impotent Kouji.
Marquis Janus is a witch more than willing to befriend people under disguise before poisoning them. Another of her acts in the Gosaku Ota manga episodes is slicing Misato in half to pay Tetsuya back for defeating her in a fair fight.
Great Marshall of Darkness is downright cruel, especially in the Ota episodes: His first strategy was blowing up one city and then warning he would do the same thing in Tokyo unless the Japanese Government destroyed the Fortress of Science and Great Mazinger. The Government caves in - and Great Marshall razes Tokyo to rubble anyways. Another of his plans involves creating a Kill Sat to blow up from the space any target he feels like annihilating. You can say Dr. Hell became an even greater bastard after dying and being brought back to life as a servant of Mykene Emperor.
King Vega's idea of what to do when your homeworld is becoming unstable and on the verge of destruction is to begin conquering and colonizing another worlds. Original inhabitants of a planet are usually exterminated, although in some instances they are "only" enslaved. To put his mindset in perspective, when his own daughter Rubina asked him why he isn't willing to make peace with the people of Earth, he couldn't come up with an answer.
Barados deliberately targets women and children in order to exterminate the Fleed's population, which is without mentioning the "let's drop several hundred of children from three thousand meters from height" -incident.
Jewel Princess from the updated series of Cutey Honey is a Psycho Lesbian who runs a jewelry shop as a front for entrapping beautiful girls. When she finds one she wants, she has her brought to the basement, stripped naked and subjected to a gas that results in the girl being turned into a statue, so she can preserve their beauty and keep them around for her to admire. She even loves their screams of despair and pain as they're converted into statues. She forces the captive heroine Honey to watch as she does this to an innocent girl, and once Honey bests her in a fight, Jewel Princess takes a serum to change into a monstrous creature to simply rend Honey apart that way.