Villains who are Pure Evil, also known as Complete Monsters, are the worst of all villains, a villain who is completely atrocious and irredeemable. 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 Steele exist if the line they cross is heinous enough.
• They are taken seriously, causing fear, revulsion, and hatred from other characters in the story. Therefore, villains that fall under Comic Relief 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.
• They are completely beyond redemption and absolutely nothing justifies their heinous crimes.
• They display no sense of remorse towards those who they have hurt nor do they express any empathy for others as well.

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 an obviously evil character such as a Serial Killer doesn't automatically make the villain Pure Evil unless they are particularly brutal in nature.