Elder Toguro, the unpleasant elder brother of Younger Toguro, makes his Establishing Character Moment by crushing two birds that Yukina cared for deeply to death in front of her eyes.
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!!!"
Or what he did to Makihara, aka Gourmet, after taking over his body and forcing Gourmet to kill himself by smashing his face into a mirror repeatedly. And enjoying it thoroughly.
He got what was coming to him twice by accidentally pushing Kuwabara and Kurama's buttons. The second is a Fate Worse Than Death as he's forced to fight an illusionary Kurama incapable of dying, while being drained of his infinite life energy by a scary parasitic plant that Kurama planted within his body.
Rando. He's a demon who has learned the techniques of 99 masters and killed them after they were no longer useful to him. Then he practices his new skills by testing them on humans, causing numerous disappearances. All of this villainy is off-screen, but in Genkai's Tournament, you get to see his cruelty first-hand. After revealing his true form, he fights Kuwabara and shrinks him using a curse. He then plays with Kuwabara by breaking his arms and legs and throwing him around like a ragdoll. An enraged Yusuke fights him, but is paralyzed by a goo he secretes. Then he throws Yusuke into the swamp and unleashes demonic fish on him.
GonzouTarukane was as bad as Elder Toguro. He tortured Yukina just to profit from the jewels created by her tears. He almost 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 is also the one responsible for founding the Black Black Club whose amoral practices caused Sensui to snap and turn against his fellow humans. What's even more disturbing, and a great emphasis on the more, is the fact that Yukina reveals that Ice Maidens shed a very special jewel when they give birth to children. The mere fact that if Tarukane EVER found out about these gems, well, you do the math. DON'T SAY I DIDN'T WARN YOU. His commupance {losing all of his money thanks to Sakyo, Getting the crap beaten out of him Hiei, who is Yukina's older brother, going insane from losing his money, and getting killed by Younger Toguro under Sakyo's orders} is very satisfying to watch.
But the member of the Black Black Club who really started Sensui off on his rampage 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 the one Sensui came across. Next thing you know, he wants to open a gate to the Demon World to just to see what kind of interesting environmental changes would occur due to this. 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.
Kazuya, one of Shinobu Sensui's seven split personalities and the manifestation of his brutality and violence. 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 animalsan 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...
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. And then Hiei sticks him in a plant that will let Mukuro permanently torture him and it is awesome.
Although utterly small potatoes compared to the above examples, Mr. Akashi and Mr. Iwamoto get a dishonorable mention on this page as well. Both make no secret of their disdain for Yusuke, even going as far as to mock him at his own funeral. Petty? Yes, but it gets worse. After getting fed up with Kuwabara's gang getting into fights, Mr. Akashi forces them not to fight under pain of revoking their after-school work priveliges, knowing full well that one of them desperately needs the cash to support his family. When Kuwabara honors the deal, suffering repeated beatings from people he's previously pissed off, Akashi forces him and his friends to score at least a 50 on their next exam. When Kuwabara succeeds at this, he, at Mr. Iwamoto's suggestion, changes one of Kuwabara's correct answers as an excuse to fail him. Fortunately, after Kuwabara restrains himself from giving the guy a well-deserved Megaton Punch, Mr. Takanaka, who overheard their scheme previously, chewed Akashi out and forced him to change Kuwabara's answer back.
Additionally, Mr. Iwamoto, while admittedly under theinfluence of the Makai Insects, tried to murder Keiko. Although it was done at Suzaku's command, the other controlled humans can't speak in coherant sentences and mostly growl, whereas Zombie!Iwamoto rants and raves about Keiko's association with Yusuke and "tarnishing of the school's image" as reason enough to kill her, which shows that there is still some of himself in there and that the insects are merely removing all inhibition.
