An interesting case in Dino Attack RPG, Rotor somehow crossed his moral event horizon and actually managed to return. Exactly when it happened is debatable, whether it was his Kangaroo Court or later the fact that he tortured a child via No-Holds-Barred Beatdown. Either way they both seemed to show that he was irredeemable... at least until he got his act together during the final battle.
In the New Dawn RP-verse, there are truly many depraved villains, but none of them cross the line like these loons:
Grahf Mordes, sacrificing his family for power. What really sealed it was his reaction to their screams and cries of "Why?" He merely laughed.
Qord was pretty insane before, but what made it really, really clear he was utterly morally repugnant was his teleporting in a young police officer just out of academy and telling an assembly of young magic users to torture the officer to death as they see fit, when he is viewed by the magic users as a teacher. What sealed the deal in this case is when the cop, in a nod to Reservoir Dogs, says he has a family waiting for him at home, Qord tells him that makes it all the better. Even the other Esoterica Higher Ups were sickened. Yes, even Zenobia.