Original: https://villains.fandom.com/f/p/3179493162221867156
Alright, so Adventure Time had finished its 8-year run. No one in the finale counts IMO, but there was one that looks like they made the mark. Now, the main antagonist of Adventure Time is an undead sorcerer who aspires to not only destroy all life in Ooo, but the entire multiverse for good measure.
As for whether he's heinous enough is not the issue. Some of his worst attempts at destroying all life include trying to have a cosmic entity named Prismo grant his wish that being the extinction of all life in the multiverse. After that failed, he overhears Finn's plan of trying to get his father who was being imprisoned in a citadel, but they needed to murder a cosmic being in order to infiltrate the citadel. The Lich kills Prismo and then offs his human self. He then tries to break all of the dangerous inmates of the Citadel out so that - in his words - he could command a great and terrible army to extinguish all life one planet at a time. So, heinousness-wise, the Lich has no issues there.
What could be problematic with the Lich, however, is just exactly what the hell he is. The Lich is a very enigmatic being that at first seems to have inconsistent explanations shot his way. Many speculate that the Lich was once an ordinary human pre-Mushroom War only to become the way he is from heavy mutation. However, the Lich is also known to transfer his soul, so the original body that he had was most likely not his. Then we have the Catalyst Comet, a comet that is known to have struck Earth every thousand years. In one of its returns, the comet crashed to Earth, causing a massive wave of death, killing the dinosaurs. The episode then suggests that the Lich was made from the essence of the comet (you can read more about it adventuretime.wikia.com/wiki/Catalyst_Comets here). It also goes into discussing reincarnation and all that jazz.
So, the big guns are in the series finale: the series had been building up to GOLB, a malevolent interdimensional cosmic horror who is the physical manifestation of discord. Now, GOLB himself doesn't qualify because he is made of evil, and hardly seems sentient enough to begin with. That, and as the physical manifestation of evil, GOLB exists to keep balance with good. One of GOLB's abilities is using its breath to transform anything around him into nightmarish abominations. There is the suggestion that the bombs used in the Great Mushroom War were made from GLOB's breath, but that's unconfirmed as far as I can tell. Given how the Lich was believed to have once been a man, it could possibly mean that he was hit by one of the bombs containing GLOB's breath and was transformed. As such, that places the Lich's moral agency entirely in question.
On the other hand, there could also be the argument that despite becoming the way he is, the Lich still had some say in choosing to devote himself to destroying all life. Villains like the Lord Commander from Final Space or Twin Masters from Hero: 108 were all transformed in some way, but nearly every time, they exemplify that they could still make cognitive decisions, but they just loved the new powers that they acquired. Maybe this is the case with the Lich since there is hardly any background information on what he was like initially if he was ever human to begin with. I'm personally going to wait around a bit before I make a decision since one of my friends is an expert on the show, but those are my thoughts as for now.
I'm not suggesting that we cut the Lich, but that we'd have more to go buy before we make any final decisions.