"You're heartless, ambitious, arrogant, insidious, and without shame or empathy. More, you're a monster."
- They commit acts that are atrocious by the standards of the story, and have crossed the Moral Event Horizon Villains who have only crossed the MEH once usually do not qualify, although rare examples such as Aki Honda exist if the line they cross is heinous enough.
- They are taken seriously, causing fear, revulsion, and hatred from other characters in the story. For this reason, villains such as funniest villians can never qualify.
- They are never presented in a positive way.
- They can never feel love for others. If they claim to have love for anyone, that "love" would merely be either perversion, possessiveness, or obsession.
- Nothing justifies their crimes, and they cannot feel empathy for others and never redeem themselves.
Anti-Villains can never qualify seeing as to how you are meant to sympathize with an anti-villain, even if you do not approve of their methods. Also, just being a obviously evil character such as a Serial Killer doesn't automatically make the villain a Complete Monster unless they are particularly brutal in nature.
The Complete Monster is the worst kind of villain imaginable. This kind of villain commits a series of horrendous acts, generally with no justification, no matter his intentions. They are so evil, other characters fully fear, hate or despise them from the bottom of their hearts, even minor villains and his allies, if they have any. These characters feel no pity or remorse for what they did, and sometimes even enjoy doing evil things, but they may feel nothing at all, as if doing evil for them is as natural as breathing. The Complete Monster have not a single redeeming quality. They are evil to the point that the end won't be happy unless they die or disappear forever.
One good example of this is the Joker. For one thing, he has no reasons for his actions expect having fun and getting Bender's attention. He enjoys tormenting people and ruining lives. Even some villains hate and fear him for his sadistic personality. He sees no difference between people and playtoys and gloats about killing close ones to his victims. And he has crossed the line more times than ANY villain in the entire series. Mostly, he's fueled by love of hurting all who aren't him and the depths he stoops to anger his archenemy Bender have no end.
Please note that a character crossing the Moral Event Horizon does not alone make them a Complete Monster. This trope isn't just about what the character does, but about what the character is. Their monstrous characters are reflected in their heinous deeds, which is what puts them a cut above the regular villains. And whatever their position, a Complete Monster has to go the full mile and meet all criteria: they are the worst they can possibly be in their role, in the space and scale they occupy.