There's a reason that almost all of HP Lovecraft's stories are in fact short stories instead of novel-length tales (If I counted correctly, Call of Cthulhu is only 26 pages long). Horror works best if it's something that maintains an element of the unknown and successfully builds suspense; if a person were to make a horror RP, it would have to be very good at pacing and not constantly be shoving graphic imagery down your throat, because if every single post were of the nasty insane people in an insane asylum, it loses its impact and shock value. Unlike movies and video games, which can stimulate other senses, written horror is entirely what the reader can visualize and how well the writer can portray their tale. Whereas a video game can get lazy and just throw jump scares in your face for cheap thrills (looking at you, Five Nights at Freddy's), horror needs to be crafted well in written form to be effective. What's scarier than the monster you describe in graphic detail is the monster you describe in very minor details, hinting at what it is and what it's capable of. Ideally, whatever the reader thinks up is far more terrifying than anything the author can describe.
I honestly wouldn't find what you're describing as a good horror roleplay as a very effective one. If you're constantly being subjected to graphic descriptions of people writing shit on the walls in their blood or eating other patients, then it just becomes a parody of itself. There's no suspense, suspension of disbelief, or immersion. It stops being disturbing when you go 50 posts and people are just trying to outdo each other for shock value. Gore and graphic imagery can be very effective, but it can't be means to an end.
For instance, I'm doing a horror 1x1 right now where my character is a police detective investigating a series of grizzly, inhuman murders in a small tourist town and while the crime scenes are graphic and disgusting, there's an air of "What could have done this?" about it since I have characters unable to agree on what methods could have been used, and then there's a fear of where the killer/ monster is going to strike next. Right now, my character is staying with my partner's character as she was a potential witness for whatever it was, and so far there's only been brief glimpses of whatever the creature is and the sounds of it trying to get into the house. Neither my partner or I agreed exactly on what it was, and we're kind of filling in the gaps as we go, and honestly a lot of what we're writing is somewhat chilling because we're letting each other's imaginations fill in the gaps. The best part is, even after this particular monster situation is resolved, there's a larger plot at hand that's going to get progressively creepier. If we didn't have any build up to the appearance of this monster and the larger implications of it, then it wouldn't have been nearly as spooky.
Think of Jaws, for instance. The movie single-handed created a cultural fear of sharks that continues to this day, and a big reason for that is because you rarely actually see the shark, but you know it's out there, and because it's concealed by water, you don't really know who's going to die next. I can't remember how much of the actual film is the shark actually visible, but it's not much. It's kind of like Jurassic Park (the original) only had 11 minutes of dinosaur screen time, but it feels like much more because the pacing is fantastic and when the dinosaurs do make an appearance, it leaves an impact. With the first Alien movie, you rarely ever see the full Alien, because it's just a guy in a rubber suit, but using darkness, clever camera angles, and great practical effects, it scared the shit out of people when it first hit theaters even though it's barely visible in the movie, either.
Point is, try not to show so much that your own monster ends up looking like a guy with a rubber suit. Let your readers' imagination do the hard work for you.
