Yu-Gi-Oh! has Yami Marik, a split personality off of the original Marik. Literally the first thing he does upon his original creation/release is kill Marik's father brutally by skinning him alive with a bronze knife within the Millennium Rod, then immediately try to do the same to Marik's only friend, Rishid, though he was thankfully stopped when Rishid restrained his evil. When he got loose again during the finals of Battle City, he imprisoned Mai and the real Marik within labyrinths of their own loneliness (and in Mai's case, this is after severelymind-raping her in their duel), attempted to murder Rishid several times, nearly killed Marik's older sister Isis, was knowingly roasting people with attacks from the Winged Dragon of Ra, set up a shadow game meant to actually kill the loser, used Marik's soul as his life point dispense (meaning that if he lost, his opponents would have to watch as the real Marik's soul was slowly eaten away), AND tried to kill the Pharaoh that is destined to save the world. What a dick.
Mr. Ishtar, Marik's father, is arguably a Complete Monster. He obviously has no love for his family and is really just using Marik in order to continue the Ishtar tradition of protecting the Pharoah. The way he treats Odion (seeing him only as a servant instead of as his son and threatening to kill Odion for any failures on behalf of taking care of Marik and was not above killing him for breaking the clan's rules) and how he tortured his own son by carving hieroglyphs on his back at the inituation against his will is absolutely abominable.
And let's not forget Yami Bakura, the evil alter ego of Bakura, who is the reincarnation of an Egyptian bandit who fought the Pharaoh/Yami Yugi. His first acts included attempting to kill/ send Yugi and his friends to the Shadow Realm in order to possess the Millennium Puzzle. He, of course, loses, but comes back soon after to attempt to use Mokuba as a new host until Tristan saves him. In Battle City, he enters the competition by sending three Duelists to hell/Shadow Realm after wining the duel with much satisfaction. His most evil acts come in the final arc where, as the bandit, he desecrates the Pharaoh's father's grave, kills innocent people to boost his power, kills the Pharaoh's best friend/most loyal servant, and revives an evil god to destroy the world. Despite his Freudian Excuse, it's hard to forgive him after this.
His Freudian Excuse doesn't even hold alot of water in the present day, as he's become a servant/vessel for the great demon god Zorc, and cares more about following Zorc's will for the sake of the world's destruction rather than revenge on the Pharaoh.
In the pre-Duelist Kingdom "Monster World" arc, it turns out that Yami Bakura was trapping people into figurines while they played a tabletop RPG with him.
Brron, Mad King of the Dark World, is one even though he's only in a few episodes. He kidnaps Judai's friends and plays a card that basically kills them in front of him, eventually bringing out a Woobie, Destroyer of Worlds Judai
Divine/Sayer. When we see him duel Rua/Leo for the first time, and how cruelly he dealt with Carly, we see that he's a jerk. Then he gets worse.Turns out, only did he torture Misty's younger brother to death in a series of extremely dubious "tests" and try to turn Aki into a Woobie, Destroyer of Worlds, he framed poor Aki for the former and LAUGHED ABOUT IT. Probably the only Irredeemable villain in 5D's, huh? He gets a very satisfying Karmic Death when Misty decides to let her Earthbound Immortal literally eat him alive.
Only irredeemable recurring villain. Arc Villains Takasu and Lotten are pretty awful, too.
