The opposite of the hero is not the villain*. It is the monster.
The hero and the villain are often flip sides of the same coin. One dark and one light, they nevertheless have a connection. A common background, a common cause, a shared set of ideals…the villain has something of the hero’s, just twisted.
The monster, however, is a different beast altogether. They can sometimes be the villain, but not always; sometimes they’re an associate or a secondary villain, sometimes they’re a henchman** who lurks in the corner, exuding menace like Axe Body Spray at a junior high dance. Whoever they are, wherever they are, they are the one who does the unthinkable: sets fire to the house with the kids still inside, butchers the village even after they collected the taxes, lets the virulent toxin loose in the air recycling systems of the old folk’s space station. The monster goes toward evil—and then goes a step further.
If you’ve ever read a book or seen a movie where there was a bad guy…and then the guy that you really hated, you’ve met the monster.
Sometimes the villain and the monster are the same person. One memorable Stephen King book I read had the villain, very early on in the book, beat a dog to death because it tore his pants. It was a horrifying act, clearly defining that man as both the villain and a monster. Heroes are often said to have a ‘Save the Cat’ moment—the point in the story where they, literally or figuratively, save a cat from a burning building because they’re the hero, god damn it. Monsters can have the opposite: a ‘Kick the Cat’ moment. Or, in this case, kick the dog. The point where they hurt someone because they can.
To take a pop culture reference: in the Harry Potter series***, Voldemort is the villain, hands down. But Bellatrix Lestrange is the monster. [Spoilers coming, though if you haven’t read the books or seen the movies by now, I doubt you’re going to, so quit your fucking complaining.] She kills Sirius, tortures Hermione, and is not only responsible for the worst crime of the entire series, but gloats about it. The characters fear Voldemort; they hate Bellatrix.
And that’s the point of the monster: to make us hate. It’s an emotional investment in the story. Just like the characters we love, the ones we hate draw us in. Some villains we can understand, or even empathize with, despite their actions. But not these guys. We just want them to die. Or, at the very least, be confined to the deepest, darkest prison imaginable with no hope of parole. They become the lightning rod for our desire for revenge and we want to see them go the fuck down.
Even better: because of their nature, we can safely hate them. They have no hope of redemption. There is no saving the monsters.
Nor does there need to be. Because there’s nothing that gets your audience going like the character they love to hate.
*Or not always. Read the rest of the post, ding bat.
**Women are significantly underrepresented in the henching fields.
***Using this one because I’m reading it again.