Liz Verity writes one of my favorite author newsletters. Whenever I see it pop up in my email I usually take a break from whatever I'm doing to read it. In her last one she included a post she'd made about 12 Content Genres to Help Writers With Character Arcs. She said something that made me think: Unlike the action genre though, horror stories have an antagonist who cannot be reasoned with and embodies evil. Since reading her newsletter, I've been thinking about one word she used: evil.
Evil is such a loaded term. Since the 18th Century the main sense of the word has been "extreme moral wickedness". Up until then evil was used similarly to how we use "bad" today. Weather, for example, can be either good or bad with no morality applied. If we expand our understanding of evil to include amoral harm to humans, an entire world of character motivations opens for your antagonist (and possibly protagonist too!).
With our expanded understanding of evil, there are two broad categories of evil motivation: wicked and amoral. Wicked evil is when a character is consciously willing to cause harm to others in order to advance their goals and interests, or worse they delight in such harm. Amoral evil applies to harm caused without conscious thought; think carnivores or forces of nature resulting in harm. Nightmare on Elm Street and Predator would be examples of wicked evil while Aliens and Jaws exemplify amoral evil.
Wicked Evil Examined
Freddy Krueger's main motivation in Nightmare on Elm Street is revenge. Therefore the killing of local teenagers is done to achieve his goal of revenge on the vigilante parents that burned him alive. Beyond slaking his bloodlust, Freddy delights in mocking his victims as he tortures them in their nightmares. He is the epitome of limitless, wicked evil.
The Yautja in Predator hunt sentient life and echo "The Most Dangerous Game" by giving the creature that hunts us cold intelligence. They don't limit themselves to sentient beings like humans, but knocking us out of the top spot in the food chain presses our most primitive fear buttons. Unlike Freddy, the Yautja have a deeply ingrained sense of honor in their hunts. They refrain from killing pregnant women and will respect (and spare) prey who defeat one of their own or even joins them against prey they cannot hunt alone. What makes them wicked is hunting sentient life for sport.
When writing a wickedly evil character the author needs to be careful to make them completely unrelatable or unsympathetic. Imagine a villain who is evil just because they are evil. No motivation. Nothing that connects them to the rest of humanity. They're so evil that even in their own story they're the villain! Do you imagine someone engaging and capable of propelling your narrative forward? Or are they a buffoon cartoon character?
Even Freddy Krueger, someone who was literally born to be evil, has a motivation that the audience can understand and relate to. He's motivated by revenge. Sure, we're unsympathetic to the reason he's seeking revenge; he is a child murderer who got off on a technicality and was technically murdered by parents seeking vigilante justice (aka: revenge). I don't think Freddy's quest for vengeance is just, but I can at least understand why he would think it is.
Digging a little deeper, I can understand the why behind what makes him tick. He is the product of the gangrape of a nun by 100 psychopaths. Considering that psychopathy appears to have a genetic component, the audience has an explanation for why Freddy is evil from birth. This roots the narrative in something the audience can relate to something within either their lived experience or understanding of the world. Just because most horror villains cannot be reasoned with does not mean they lack reasons or basic reasoning skills.
This is why pet the dog or save the cat are common tropes. After all, when Ripley pauses to make sure Jonesy is safe in Alien, we become more invested in her because she's not focused solely on her survival to the exclusion of all others--to include pets. Remember that these tropes work for both protagonist and antagonist.
Speaking of Alien, let's talk about animalistic or amoral horror...
Amoral Evil Examined
Amoral evil is fun and guilt-free to write. Your antagonist cannot be reasoned with nor will it (because now we're talking about non-human monsters) care about human suffering. But what makes these characters fun and guilt-free is we're talking about the "bad weather" application of evil; they cause harm without the emotional baggage of morality. This, dear author, is how we write horror that is 100% distilled fear.
The xenomorph in the Alien franchise and the shark in Jaws are predatory animals doing predatory animal things. Their motivation is to feed, reproduce, and survive encounters with threats from other aggressive species such as humans. This level of horror operates on our deepest primitive fears that helped our distant ancestors survive outside of civilization and as part of nature. We don't need to explain why we can't reason with a xenomorph or great white shark because the audience already knows that a person cannot reason with an animal. Nor do we need to explain motivation since the need to feed, reproduce, and survive is a universal for all living beings.
But not all animalistic villains are created the same. I would argue that the shark and xenomorph are two different subtypes of this archetype. The shark is pure animal and the xenomorph is pre-sentient.
With the shark in Jaws there is no intelligence behind what it does. It comes across humans, finds us tasty and decides to stick around Amity for their people buffet. This is a pure human vs nature tale. The shark represents the chaos and randomness in nature's oblivious disdain of our sense of superiority to it. (If you want to get into the shark's head read the novel and play the Ravensburger table top game.)
On the other hand the xenomorph is different in that it demonstrates an ability to learn and displays rudimentary thought. The Queen operates elevators and has learned to target power lines. While the species look like bugs, the Queen's desire to protect her young (and anger when her eggs are destroyed) demonstrate a maternal instinct that is mammalian in nature. These allow the character's creators to play around with the fearsomeness of intellect decoupled from morality.
In my own writing
Jaws and Alien both play a role in the creation of my first creepypasta on Fear From The Heartland. Pea Ridge is a trilogy (with a bonus Christmas Special) centering around an endangered species of pre-sentient predator being moved from a dying world to a galactic endangered species refuge. The Creature hunts telepathically targeting the sick and dying. When the spaceship she's being transported in crashes on Earth, she's suddenly aware of the smorgasbord of human emotions.
So if you're sad and depressed you're on the menu.
And it's been fun writing the Creature! She's a complete threat to humans and views herself as above us on the food chain. But there is no malicious wickedness. My current novel in progress is turning this trilogy into a novel (which is a first for me: writing at the request of an audience!) and I've really enjoyed developing her motivation and character. I get to explore things like the evolutionary origin of religion and the religious behavior of elephants.
All without the constraints of human motivation and morals.
In conclusion
What is considered evil has evolved through time. Today the most common definitions are centered around wickedness. But if we expand our definition to all harm and not limit it to morality an entire universe of motivations and characters open up to us. One that is not limited to horror.