I have a new candidate: The Master (Scary Godmother)
What's the work?
Scary Godmother is a comic series that eventually spawned two TV Halloween specials. It is made for kids. This guy is a shockingly vile villain for such a lighthearted work.
Who is He/What Has He Done
The Master is the oldest existing vampire. He runs a "Summer Ghoul" camp to teach fledgling vampires how to hunt, but this is just a scam to get him slaves to do his bidding. He forces the students to dig graves and hunt for him while he lazily sleeps. He eats most of the prey himself and keeps the students starved. He also hypnotizes the carrier animals used to deliver messages from the families of the students, making the carriers lie that everything is going fine. When a student caught him hypnotizing his carrier wolf, and confronted the Master about it, the Master forced the boy to compete against him in the "Trials of Transylvania" graduation ceremony, an obviously rigged race that pits the malnourished student against the completely healthy Master. When the student failed, the Master locked him outside of the camp's cave, leaving him to die in the sunlight. When the boy's sister begged the Master to save her brother, he pushed away the boulder blocking the entrance and thrust her face into the sunlight, permanently scarring the girl as her brother burned to death.
When Orson arrives at camp, he discovers the situation, and is soon made a slave like the other students. While out on a hunt for the Master as part of his "survival training", he happens upon his human friend, Hanna. The Master appears and wants to eat Hanna. Thinking quickly, Orson suggests they take her back to camp to show his catch to the other students. The Master agrees.
After they return to the camp, the Master begins to talk to the other students about eating Hanna. While he's doing this, Orson and Hanna quickly discuss a plan. Orson than tells the Master that he wants to be the one to eat Hanna since he caught her. Enraged, the Master threatens to eat not only Hanna, but also Orson., revealing that he is a cannibal Orson challenges the Master to the Trials of Transylvania, with the winner being allowed to eat Hanna and the loser burning to death in the sun. The Master of course wins the race, but Orson uses Hanna's sunscreen that she had rubbed over him while the Master was talking to ttthe other students to survive in the sun. He convinces the Master that he has become stronger than him and is thus immune to the sun. He scares the Master down into a grave, and he and the students seal it up.
After Hanna reveals that she is not real prey and is friends with Orson, and after they explain the trick they pulled off to the other students, the kids get some bats to deliver messages to their families informing them of the situation. The families arrive and rescue the kids, but it's discovered that while the Master was underground, he managed to escape by digging a tunnel. While his scheme is thwarted and his true nature exposed, the Master manages to evade punishment for his crimes, escaping and being left a fugitive.
Freudian Excuse/Mitigating Factors
It's stated the Master was not always the evil monster that he is now, but years of isolation drove him mad. Sad maybe, but his actions are too atrocious to make him a tragic villain. His villainous actions are motivated from his laziness and desire to get plentiful food without having to do any work, which is why he uses his charges as slaves. He keeps them weak so they won't rebel and because he does not care for them. He represents the sins of gluttony and sloth in their worst forms. He kills students who realize that he plans to use them until they die so as to keep this from getting out to the other students. While it's stated he went mad from isolation, he's clearly sane enough to understand that what he is doing is wrong, he just does not care.
He enslaves children and keeps them weak and starved, forcing them to hunt for him. He intends to keep the students as his slaves until they die from the harsh work environment. He left one student to burn to death in the sun because he got too close to figuring this out, and he burned and scarred the face of the boy's sister. He tries to eat Hanna, a young child, and is willing to eat Orson.
The comics and tv specials are for kids. The specials don't have any actual villains, and the only other villain in the comics is Tinkaree, who is an anti-villain that eventually sees the errors of her ways and reforms. He easily stands out as the only truly evil character in the mythos.
He is one of the most gruesome game changers I have ever seen in any media, period. An easy keeper in my opinion.