American Dragon: Jake Long is a cartoon that ran from 2005-07 for two seasons on the Disney Channel. The story followed a boy named Jake Long from New York City who has the ability to transform into a mighty dragon, and uses his abilities to fight evil and protect a secret world of magical creatures. His archenemies come in the form of soldiers known as the Huntsclan, who sought to eliminate all magical creatures on the planet. They are led by a mysterious tall figure known as the Huntsman.
Who is the Huntsman
I'll try to work on this one since I haven't seen this cartoon in decades.
The Huntsman started out as a weakling among his Huntsclan apprentices, and was mocked by his peers for his squeaky voice. Then one day a man fell in love with a woman who could shapeshift into a dragon, conceiving a human/dragon child named Jake Long. The Huntsman's first assignment was to kidnap the patriarch, Jonathon Long, and use him as bait to reveal his wife's Susan's true dragon form. However, hijinks occur due to modern Jake, having recklessly used his ability of time travel, with the Huntsman failing to slay him. Jake accidentally drops the Huntsman into a large pit, the hideout of a massive beast. The Huntsman managed to escape, but was left badly scared from the beast's mauling and his squeaky voice became deeper and darker. From that day on, the Huntsman developed a sworn hatred for all magical creatures, dragons specifically, and sought to destroy all of them. His plan was to collect the thirteen Aztec Skulls, artifacts built by the Aztec Empire, as collecting all of them will enable him to make a wish.
Throughout the years, the Huntsman rose up the ranking of the Huntsclan to become the leader of the clan. He would kidnap numerous children who had a dragon birthmark on their bodies (the Mark of the Huntsclan) and educate them into believing magical creatures were "lesser beings" and why they should be ridden from the planet. Any student who failed to meet his expectation would have to prove their worth by being forced to battle and defeat a man-eating kraken. The majority of whom have not returned alive. One of his students was Rose, who was snatched away from her parents as a newborn and was raised into believing the Huntsman was her uncle with her parents being deceased. He chose Rose as his apprentice, becoming known as the Huntsgirl.
Throughout the series, the Huntsman and the Huntsclan battle against Jake Long and dragon allies in gathering the Aztec Skulls, with the clan gaining the upper hand holding ten of the skulls. Eventually, during one fight with Jake, Huntsgirl/Rose learns the truth about her origins and turns against the Huntsman. Unamused, the Huntsman reveals he knows exactly where her real parents are and orders Rose to steal the last three Aztec Skulls held by Jake, threatening her that if she fails or refuses, he will kill them. Rose sadly accepts and is forced to betray the dragons to obtain the Aztec Skulls. With all thirteen Aztec Skulls in his possession after successfully defeating Jake and the dragons, the Huntsman begins his ritual to destroy all magical creatures. But before he could finish his wish, Rose knocks him aside with her staff and wishes for the destruction of the Huntsclan instead, resulting in all the evil-doers vanishing. Rose also begins fading, but is saved at the last second by Jake who wishes she was never taken by the clan. All that's left of the fourth biggest threat to the magical world was his cape.
Attempted genocide on all mythical creatures, stealing children to become his soldiers, and ordering Rose to follow his commands or risk the death of her parents. All because he was once a bullied weakling. Right...
Out of all the villains Jake fights in the series, the Huntsman stands out as the worst. Most of his enemies come in the form of random Huntsclan members who do little to stand out or mythical monsters that are simply doing what they do to survive. Meanwhile, the Huntsman nearly succeeds in committing mass genocide, has multiple innocent people under his victim list including children, and completely stands out from a comedic cartoon with very little if nothing humorous about him.
Any other heinous candidates? There's the Dark Dragon who plotted another genocide, this one on humanity, so he could wipe out humanity for the dragons to prosper (despite having killed dragons who tried to stop him). But the Huntsman stands out more since he was behind kidnapping children to succeed in his goal.