Complete Monster

The Worst of the Worst. (This is YMMV, so characters the author considers monsters.)
NOTE: The works could have tonal issues by being created poorly. So, the "Played Seriously" part is judged by how seriously they're played in the work itself.
4 vs. Satan:
*NONE
Alex in Princess Quest:
*NONE
The Stone of Power:
*"The Stone of Power 2" has the Big Bad Duumvirate:
**Rawn is the son of Artimus, but with none of his Remake incarnation's redeeming traits. During the game, he has one of his dragons attack a town, killing said dragon when it fails him. He also has two of his friends massacre a castle, with both incidents leaving a handful of survivors. He also intends to use the dragon to rule the world. During the climax he admits he did everything for power and even the three previous villains, except Artimus, end up assisting the heroes to stop his madness.
**The unnamed green dragon is a long-lived being, who lives by slaughtering as many people as possible. In the past, 500 years ago, he wiped out a town before going back to his cave. In the present, on Rawn's call, he still intends to cause said damage. Even after being freed, the dragon goes back to his destructive ways and announces his intent to do it to the entire world.
The Three Kingdoms
*Vlados Palerius one of the masterminds of every atrocity committed in the game. In order to get rich he has cities massacred. He even has one of his men working for him by kidnapping his loved one. When King Charles IV. plans to negotiate with him, he attempts to have him executed admitting his people being just his pawns and wanting to create his massacres no matter what.
Dragondawn
*The Three Dragons of the Apocalypse are the world's most vicious monsters. Their Establishing Character Moment has them wipe out a town and then one of them destroys the heroes' headquarters and then slaughters the detective and the remaining knights in the forest, admitting it kills people because it hates them. Another one, before the confrontation, also massacred two towns, as stated by one of heroes. After the lesser two are killed, the remaining one is revealed to have destroyed an entire island and shows no sadness after being informed of his fellow dragons' demises and, if the heroes fail to kill him, has their entire world wiped out, leaving them alive just to spite them. Even when they're supposedly avenging their creator Hades, they not even once mention him, being simply in it because they love destruction.
*Hades is the treacherous member of the six Gods responsible for the world's worst misery. In order to have the world for himself, Hades intended to backstab his fellow Gods, killing four of them. After the remaining one, Loki, sacrifices himself to stop him, Hades turns out to have created the three dragons above, so millions of innocents will be eventually killed, should he fail to rule over this world.
Harter Knall
*King Greyland is greed personified. After getting the throne, Greyland raises the taxes and has anyone who can't pay him killed and killing his two advisors for objecting his methods. He later plans to have the revolutionaries killed despite them being almost children and the S6-unit is only following him in killing every attempt at revolution because they're terrified of his army.
Hero Story
*Master Shevu is the leader of the demons. After a war ends in a stalemate, Master Shevu has towns massacred for his demons to conquer. After Zackles kills his follower Derecon, Master Shevu sends his demons to massacre another world. When finally confronted for his actions, he admits he intends to turn the human world into a world of demons because according to him only the strongest have a right to exist.
Kill the Demons
*NONE
Satan Saga
*Satan is the highest demon lord planning cause as much mayhem as possible. In the past, he killed Trianuk's wife before making him to have killed everyone so he can avenge his wife having already killed the bounty hunter he send to kill him, with the entire castle massacred as result. And even after Trianuk finds out the truth, Satan leaves him alive so he can watch the man massacre other bounty hunters. After Trianuk is killed, Satan sends his people to massacre other towns and when confronted by the heroes, Satan admits he did everything for fun and because he likes blood baths.
*Meynard the destroyer of towns is one of the two infamous criminals wanted for their actions. Before his debut he sends demons to destroy a town and after joining Trianuk wipes out Roy's town. Then he proceeds to slaughter a orphanage, maniacally laughing afterwards. Short after he teleports the king into a gas chamber to be executed by the Pope, shocking the Pope. Then he proceed to backstab Trianuk after the latter gave the throne for him. Even Satan refuses to hire him due to Meynard's backstabbing nature. When the heroes confront him and his top-enforcer Walter the two brag how they enjoy doing evil before fighting the heroes. After Walter fails, Meynard tortures and kills the guy, laughing sadistically. Even when he's about to be executed by that gas chamber, Meynard remains unrepentant to the end.
*Among the later enemies our heroes face, King Greynor is easily the worst. After finding out about the Pope's massacres, he promotes Dagey to be his general and then has him executed. Short after he has his own kingdom wiped out so people won't remember the Pope's massacres and then plans to do the same to the kingdom Dagey comes from.
Story of Hero:
*Elizon Tataros is the leader of the after him named Tataros-cult, who desires to become the king by taking the throne from his brother. His first act is to kill a emissary just to warn his brother. When one of his minions fails to get one of the books, he tortures his right-hand man for failure. Later, he sends his minions to get his throne, burning down the castle and torturing his brother to death. During the climax, Elizon summons Titan to destroy the castle again, planning to rebuild it, uncaring about the many people he'd kill by doing so.
The Legend of Devils
*NONE
The Sidestory Duology:
*NONE
The Time Of Bloody Fear:
*NONE