These are the tales of the Skeleton Warriors... (epic guitar riffs ensue)
What's The Work
Skeleton Warriors is a cartoon from 1994 that is centered around the main threat, the Skeleton Warriors, and the protagonists, the Legion of Light (composed of Lightstar, Talyn, Grimskull and Guardian), who try to stop them from fully taking over and corrupting their planet, Luminaire. The series spawned a toyline, trading cards and a short comic series by Marvel as well.
Who is the Candidate? What They Did?
Baron Dark is the series' main antagonist and the leader and creator of the titular Skeleton Warriors, as well as the first of their kind. He was the king's advisor and used this position of power to manipulate others to his advantage, as well as approach the king when he least suspected and assassinate him. He manipulated the king's sons into a feud and nearly obtained the Lightstar Crystal which would grant him omnipotence. Instead, he ripped it in half and was stripped off his flesh.
He became a skeleton with the power to corrupt others into monstrosities like him so long as they had enough evil in their heart, and with the inherent immortality and indestructible bones he had gained, he quickly took over most of Luminaire, turning the entire population into the diabolical Skeleton Warriors and imprisoning or executing those who couldn't be corrupted, or using them to trick the Legion of Light.
Dark is very open regarding his evil. He enjoys tormenting innocent souls and loves seeing chaos and bloodshed spread, and though he can no longer feel anything, he wishes he could to know how well it once felt to witness a landscape of death and carnage. His most common victim is Grimskull who shares a mental link with him thanks to the splitting of the Lightstar Crystal, and Dark constantly uses this power to make Grimskull's mind a living Hell, turning his dreams into torturous nightmares.
His one true motive is to remake Luminaire and its inhabitants in his twisted and malevolent skeletal image, a goal he will achieve (and comes close to accomplishing) with the complete Lightstar Crystal.
Freudian Excuse/Mitigating Factors
Described as being the very embodiment of evil, Dark lacked any goodness in his heart even when he was human. The only "mitigating factors" I could add here are the fact that is the descendant of ancient dark lords and that he felt it was his birthright to rule Luminaire like his ancestors, but it was his choice to do so, and he could've simply lived a nice life with his family and kin as the king's trusty advisor. Instead, he chose to kill or corrupt them all.
This is revealed in supplementary material, but in the cartoon none of this is said, leaving Baron with very little reason to turn the entirety of his world into monsters beyond, in his own words: "I can".
Similarly, despite Dark's offer to Grimskull to join forces with him and to be the brother his own sibling never was, this is just a ploy to get the other half of the Lightstar Crystal, since in reality he thinks Grimskull's just a pathetic youth.
In the corrupted world of Luminaire, it doesn't get any worse than Baron Dark, and anyone close to him in evilness (like Dr. Cyborn) works for him.
An irredeemeable monster, even long before he became an immortal skeleton, and a keeper, I think.