Naraku from InuYasha. Despite the gigantic laundry list of horrible crimes he's committed, they're downplayed by certain sections of the fanbase because of his "nice looks". He's technically a Karma Houdini for most of them. All you can do to him is kill him. He doesn't have any ties to any other person, human or demon. But what he can do to you...And to think he was this bad BEFORE he was Naraku. Becoming Naraku just gave him the power to be as Jerkass as he ever wanted.
Driving the point home, there is a reason that the heroes spend a good portion of several arcs chasing after and trying to kill a small child and a baby. Hakudoshi is a full on psycho who went around slaughtering Youkai to make Mouryomaru for Akago to use. Akago is worse: his powers include the Hannibal Lecture and More than Mind Control. He plans on forcing a Face Heel Turn in Kagome when she's in an extremely low emotional moment, and he is plainly manipulative and foul.
Most of Naraku's offspring (aside from the aforementioned Hakudoshi and Akago or Juromaru and Kageromaru) are anti-villainous woobies that die peacefully - or at least redeem themselves before they die. Not so for Goshinki, Naraku's psychic homicidal offspring that murdered an entire village of innocents justto attractInuyasha's attention. And when Inuyasha steps into defend two newly made orphans, Goshinki breaks the Tetsusaiga and smashes him out of the air. Nobody cries when Demon!Inuyasha slashes him into a spray of gore on the landscape.
The fake water god also counts, with his entire thing of forcing people to give him their kids for him to eat so he will not destroy their village.
And Shiori's bat youkai grandfather, who heartlessly uses and manipulate the little Barrier Maiden for his own purposes and kills his Noble Demon son, aka Shiori's dad, when he questions him.
Don't forget the Ancient Noh Mask, which eats flesh and wants the jewel shards.
Magatsuhi, the demon sealed within the Shikon no Tama itself. It's literally evil incarnate.
Mukotsu of the Seven-Man Army. In the anime, he was softened considerably, but in the original manga, he was a depraved misogynist, explaining to Kagome and Shippo that he loves to kill women with his toxic gas (although he clearly doesn't mind too much who else dies, as evidenced when he annihilates an entire village to demonstrate his poison), and then attempts to rape Kagome.
