Complete Monster/Yu Yu Hakusho


 * Elder Toguro, the unpleasant elder brother of Younger Toguro. In contrast to his honorable Noble Demon younger brother, Toguro freely admits to being a treacherous sadist devoid of honor. He makes his Establishing Character Moment by gleefully crushing two birds that Yukina cared for deeply to death in front of her eyes. His repulsive character is further displayed in the Dark Tournament, where he spares two fighters and promises to let one join Team Toguro: when one begs for his life, Toguro kills him...and then kills the other because he just hates keeping his promises. And he relishes in this savagery towards his opponents the whole way through.
 * The Abridged Series even lampshapes this with the line "So, let me get this straight, you're paying us out of your own pocket to make a little girl cry? ...I F*ING Love BEING A DEMON!!!"
 * Three words: "Genkai puppet show". Used to make a mockery of the woman's life and death, and to break Kuwabara's spirit, this display is so repulsive that his brother,, literally kicks him off the island, particularly after he admitted that he had intentions of raping her when she was younger, in the days before he even became a demon.
 * Or what he did to, after
 * He got what was coming to him twice by
 * Gonzou Tarukane, the Smug Snake Big Bad Wannabe human crime boss, was almost as bad as Elder Toguro. A Fat Bastard extraordinaire who cares nothing but for what will make him more money, he tortured Yukina the Ice Maiden just to profit from the jewels created by her tears. He almost totally dehumanized her (not that she is technically human, but still), calling her a "frigid little whore" and wondering how "a creature like her" could have feelings. He hired the Toguros to torture her and is revealed to have physically tortured Yukina in the past. After this, he had one man who tried to help Yukina executed on the spot and taunted her about it, using his death from trying to rescue her as a psychological grip on her so that she'd never try to escape him again. According to the English dub, he is also the one responsible for founding the Black Black Club . Vile, greedy, cruel and cowardly, Tarukane was the worst humanity had to offer in the series.
 * But the member of the Black Black Club who really started  was Sakyo. First endearing himself to the audience by killing off the aforementioned Tarukane, and later Smug Snake Butajiri, and, in the anime, having a romance with Shizuru, only to later reveal his true master plan: to open up a portal to the Demon World just for excitement, and revealing to the Younger Toguro his backstory: he grew up with loving parents and siblings whom he hated, he worked at a pet store as a teenager, torturing and killing animals, then became a master gambler and crime lord who often utilized torture and death in his gambles or projects, including   At the end of the day, Sakyo truly has no real good qualities within him.
 * Dr. Ichigaki also counts. Makes a kind martial arts master sick so his followers make a Deal with the Devil with him to cure said master, turn said followers into bio-engineered and Brainwashed and Crazy killing machines, and does NOT cure said master. No wonder Yuusuke was MAJORLY pissed off at the old man when he found out.
 * Not to mention that Ichigaki openly admits to not caring about said brainwashed killing machines and only entered into the tournament to get Yusuke's body; once that happened, he'd dispose of the disciples himself. AND he takes a sick pleasure in watching Yusuke and Kuwabara choose between winning the match (and killing the innocent followers) or trying to save them (and end up slain), even taunting that, due to his modifications, "their humanity is already dead!" Classic Complete Monster-dom with a healthy dose of mad science that makes his ass-kicking  all the sweeter indeed.
 * Gouki was the earliest appearing example. The only one among the trio of him, Kurama, and Hiei not to reform; he's a notorious wanted criminal in Spirit World, with murder described as his hobby, and he spends his screen time using an Artifact of Doom to suck the souls of out children so that he can eat them! Granted, devouring souls is what his demon species does but what makes Gouki stand out is that it wasn't good enough for him. He doesn't like having to tear out and cook the souls - he wants to eat them fresh and raw. Particularly young souls of children.
 * Kazuya,  He starts off by blasting Yusuke multiple times in the arms and legs with his arm gun, then briefly hums to Yusuke's screams of pain, constantly cackling, "You can't feel pleasure without feeling a little PAIN!" Appearing directly after the charismatically brilliant Minoru, Itsuki informs the heroes that this asshole considers brutally torturing small children and animals an entertaining past-time, though you fortunately don't see it onscreen (in the manga, Itsuki even says that, hypothetically, Kazuya would enjoy killing a defenseless baby!). And judging from the stance Itsuki takes when Kazuya first appears (he looks a lot like a Domestic Abuse victim), it is fair to say he knows the psycho's cruelty firsthand. What's interesting is that
 * And then there's Mukuro's stepfather: he outfitted her with cybernetics to turn her into his sex-slave at BIRTH', and started raping her as soon as the procedure was complete. This continued throughout her childhood until she disfigured herself with acid so she'd be too unattractive for him to be interested, at which point he simply threw her out. And just to make sure she wouldn't try and kill him later on, he implanted Mukuro with false memories of him having been nice to her before. Let that one sink in: he had to give her false memories about treating her kindly because he never did.