Moral Event Horizon

The moment some villains show themselves as vile and irredeemable scumbags.
4 vs. Satan:
*Devontres massacring towns and shrugging it off like nothing.
*Freezer destroying a church.
*Satan may be past it given who he is, but offing a bunch of gods to take their power seals his fate.
Alex in Princess Quest:
*Malthor van Dracula when he kills the king and the mayor to kidnap the princess.
The Stone of Power:
*Balthar when he massacres a kingdom just to prove a point.
The Three Kingdoms
*Vlados Palerius was already an awful person, but he pretty much dooms himself when he, rather than accepting peace talks, orders the king executed.
Dragondawn
*One of the demons killing the high priest.
*The dragons when one of them massacres the headquarters.
Harter Knall
*Greyland crosses it in the second scene where he orders the taxes to be rised and those who can't pay get killed and kills his two helpers who object his decision. 
Hero Story
*The hooded man (Derecon) when he massacres a town and laughs about his carnage.
*Shevu starting a war in the past that still holds even in the present day.
Kill the Demons
*The demons massacring towns. It pretty much ensures they stay bad the entire game and just end up on the receiving end of the game's title.
Satan Saga
*Meynard the "town destroyer" was probably already past it, but when he slaughters an orphanage and laughs about the carnage and then has the king killed by teleporting him into Pope Bonmos' gas chamber, he pretty much ensures he won't get forgiven.
*Trianuk killing Alex' father in an unfair battle.
*Satan admitting to have killed Trianuk's wife and then turned him into a mass-murderer for fun.
*Bonmos ordering two towns massacred when one of them hates him.
*King Greynor proves himself to be an utter hypocrite when he has the towns of his own kingdom massacred and then planning to do it on another kingdom. Not even his own guards approve of his methods.
Story of Hero:
*Elizon Tataros having the castle massacred and killing his brother because he himself couldn't have the throne.
The Legend of Devils
*N/A
The Sidestory Duology:
*Rudolf Hitler orchestrating Darion's death.
*Sez sending his wolf to massacre innocents.
*Soult killing everyone in the factory by poisoning the workers to death to create a slime army.
The Time Of Bloody Fear:
*Reika and Feeza killing Sam for hitting the second one.
*Maltros destroying a town to build his castle and later when he kills the king so he himself can become one.