I've talked before about making sure that all your characters are fleshed out. They all need backstories and motivations, and your villain is no excuse. The old saying is that "no one thinks they're the villain." That's good advice. But everyone once in a while, I think it's okay to have one character... just one... that is absolutely and totally evil.
Done right, this can make a good character. I see it usually played out in one of two ways. If it's your antagonist, then they're psychotic. They understand the pain and suffering they're putting people through. They understand that what they're doing is wrong. They just don't care. This can be used to great effect, but I fear that many writers (myself included) may lack the talent to properly utilize this. If you're off even just a little, you might end up with a cartoony villain, cackling maniacally and twirling their mustache.
The key, I think, to this trope is that, while , yes, they may be evil incarnate, they don't always need to go around advertising it. Subtlety is everything. Imagine how much more jarring and unsettling it could be to have characters slowly come to the realization that the antagonist is unfeeling, uncaring, and completely without any hope of redemption, than to simply have it announced that they're Super Space Hitler (tm.) The unnerving feeling as you see more and more what the character is willing to do, the reader finally coming to the only conclusion they can: this character has no redeeming virtues, that they exist only to torment others.
The other way I see things is if your monster is more an obstacle than an antagonist. This leans more heavily on the side of beasts and demons. Creatures that have no soul, no conscious, that by their very nature cannot be reasoned with. These creatures are forces of nature. They are evil because that is how they are made. Again, it really depends on what your monster is, but it can be easy to fall into making them too cartoony. Like the villain, I suggest subtlety.
The less you see of the creature, the more you imagine. The more you imagine, the worse it gets. Can it be reasoned with? A slow burn, revealing that the creature is mindless or souless or whatever "less" you need to convey it's motivation, I think can do wonders for building this monster into the stuff of nightmares.
So that's my take on the subject. What do you think about monsters and creatures that are pure evil? Is there always a bit of good in everything or can some things be pure evil? Let me know on social media or down in the comments!
Keep your eyes open for my debut novel, The Paladin.