While Amon in AMON is nothing like his OVA counterpart, this series introduced a handful set of Complete Monsters in his place. Candy is solely responsible for convincing people there is no such thing as Devilmen, leading to them being prosecuteed and killed alongside countless humans believed to be demons, yet he is one of them himself and isn't to shy to admit to being a Serial Killer before his rise to power. Then there is God, who orchestrated the fate of the entire world since Satan has fallen, so he/she will lose the only person he/she loves as a punishment. And put the world on time loop so Satan has to suffer watching Akira die over and over again.
Jinmen, the depraved turtle-like demon from the original manga who eats people alive to devour their souls and incorporates their faces into his shell - very much alive and concious. He has no problem taunting Akira with it. In fact, he ate somebody close to him and put that person's face above vital spot. Who he ate varies between versions - in the original manga, it was a ten years old girl named Sachiko, Akira's close friend; in the OVA series, it was Akira's mother; and in The Movie, it was an ex-bully who befriended Akira. The Movie even has special scenes of him attacking little kids he wanted to eat. Jinmen targets vulnerable humans on purpose, with children being a favorite meal, including a 10 year old named Sachiko who had befriended Akira.
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 is revealed to have been manipulating 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, who had been born both male and female, wanted to use Jun to become a new God of the world. Asuka 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 and selfishness.
Satoru, the seemingly young leader of the militant Devil Beasts, is an absolute sadist who believes in the genocide of humankind. Devoting himself to the supremacy of Devil Beasts, Satoru inspires the awakening of them as feral beasts who slaughter those around them while masterminding a series of brutal terrorist activities that claim many lives. With no patience for Devil Beasts who fail to embrace his fanaticism, Satoru sees them as enemies to be annihilated, especially Jun Fudo who Satoru dedicates himself to destroying mentally. Satoru at one point abducts and brutally murders Jun's friend/love interest Kazumi while mocking her over the death of "one human," before attempting to finally kill Jun himself. Sinister, cruel and monstrous, Satoru shows that he and Asuka are simply two sides of one dark coin.