Horror is a huge umbrella term for so many aesthetics, and while it is always a good idea to learn basics if you want to draw anything (anatomy, color theory, proportions, 3d shaping, perspective, values, even storytelling and directing if you wanna do comics!) remember that you can make a creepy/scary/disorienting pic with very little.
It all depends what specific genre of horror do you want to represent. You mentioned Army Of Darkness, so I can imagine you like the gorey maximalism of classic slasher movies and games. Jason, Freddy, Splatterhouse, and all that! I'd say it's the genre that most requires anatomy knowledge but also full of referencing for you to study on. There's a lot of beautiful movie posters, game covers and going back to Army Of Darkness, even fantasticly drawn comics you can take inspiration from!
Talking about comics, bear in mind that horror can also be parodistic, funny and slapstick at the same time. There's no need to limit yourself with dark palettes! I did plenty of horror-esque art that was full of bright and contrasting colours, their clash together gave the feeling of confusion and aggressivity.
However! You can also go to the opposite direction and think of horror as moody, atmospheric and lonely. I've seen a lot of horror-coded modern art that feature empty environments, dark halls, liminal spaces with looming shadows or bright bulbs of light as the only creature inside. Let the viewer imagine the monster rather than drawing it directly! X3