Complete Monster

The Complete Monster is the worst of all villains, a villain who is pure evil. And for this kind of villain, doing evil for them is as natural as breathing. They can be recognized by these signs:

• 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 Grand Moff Tarkin and Shou Tucker 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 Comedy Villains and Comic Relief Villains can never qualify.
• They are never presented in a positive way.
• Nothing justifies their crimes, and they cannot feel empathy 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 or a Homicidal maniac doesn't automatically make the villain a Complete Monster unless they are particularly brutal in nature.