If I were going to run a horror campaign, I'd rather run it in Delta Green (if eldritch horror is permissible) or Twilight 2000 (for a strictly mundane campaign). Both of them have plenty of reasons to put characters in situations where a significant part of the horror is the terrible things that the characters do. In the former, even in campaigns where the characters theoretically have a lot of agency (which isn't all of them; in particular, Impossible Landscapes and God's Teeth both challenge this at their very core), the demands of the "mission" may result in players deciding for instance that their characters need to severely harm people that don't really deserve it in any conventional sense. In the latter, the high lethality, the ambiguity, the distrust of strangers, and the scarcity of resources may have players agonizing over every interaction between their characters and those who may just be people in need, et cetera.
The other major reason is that the horror and bleakness is a conceit of those systems, and that the players have agreed to it if they are in those systems at all. Unless they're going in really, really blind about what those are about, the players aren't expectingto be knights in shining armor saving the day and having everyone sing their praises; fear and darkness are built in.
That sounds a lot like an admission about how a series that explores how profound trauma, constant terror, and the collapse of societal rules impact human identity and morality descend into PTSD & survival horror is probably not a great example for any sort of horror in a ttrpg just because it happens to have zombies. I dunno about you & the GM's you've met but I think it's safe to say thatPHD's aren't writing books analyzing the psychological aspects of our campaigns.... seems a few steps beyond next level GM'ing skill to invoke it as a target for the GM when the system can't be bothered to support the GM on any sort of survival themes plausible risk of death ongoing material needs fpr PCs & so on.
If it were a good target for GM's with such a deeply emotional gameplay I wouldn't need to say that love's pain has no save SR:no no attack roll & auto succeeds regardless of distance or planar differences because such a spell is in the same book at mind rape but don't need to mention the book name after namedropping that one
What you really need for an effective horror game, or fiction, is tension that builds up a sense of dread. For all the talk of Ravenloft being a pseudo horror setting, I would agree though I don't see that as a negative, the Curse of Strahd campaign managed to provide players with some seriously tense situations regarding some very sensitive subjects.For the purpose of this discussion, let's say that we’re discussing actual scary horror here; legitimate spooky and emotional stuff. I suspect the answer to “How do you do horror?” is that many tables simply don’t. They just plop PCs into a horror setting and play the exact same way they would in a medieval fantasy or magitech setting. But just because you drop your party in Ravenloft and dress all the NPCs in goth gear doesn’t make a campaign scary. This discussion isn’t about those pseudo-scary games, it’s about the True Scotsmen of horror (with self awareness).
Yup, you really, really need the players onboard so far as the tone of the campaign goes. When you say the goal is to be scared to whom are you referring to?More than most genres, horror requires an agreement from everyone at the table that they are dedicated to a horror game. Everyone has to agree that the goal is to be scared. Because building tension is real work, but breaking it can be cheap and easy.
They really are desensitized. It's tough to maintain tension when you know the stats of a skeleton by heart and when everything is fantastical it means nothing is fantastical. I think this is the biggest problem you run into with D&D an horror. In Curse of Strahd, a lot of the tension comes not from what creatures might do to the PCs, but from what might befall various NPCs. What's going to happen to Ireena Kolyana if they can't protect her from Strahd? What of the abbott's child bride?Think of staples of horror like zombies, skeletons, big spiders. These are all things a typical D&D player faces at level 1. Even werewolves and vampires eventually feel relatively mundane.
My preferred method is to ask my players if there's anything they absolutely do not want to see in a campaign before we start play. Though now that I think about it, I've never used this method when running one shot horror games.Horror games face a unique problem: peak success is a very delicate balance. It’s rare to have a problem with a comedy game being too funny, or running a game focused on fey with too many fey. But too much horror can be a bigger failure than too little horror. The horror genre pushes people to the limit of what they are comfortable with. But if a DM goes past that limit, the entire game might have to stop.
Don't forget rust monsters!The only D&D mechanic I ever saw approach "players are scared" was old school energy drain. Permanent level loss was worse that death to many AD&D players. Modern D&D does not have anything like that (in addition to focusing on much more competent and powerful heroes).
Death saves, bonus action healing word bonus action lay on hands bonus action healing light, overly generous recovery mechanics with a near impossible to fail at bar to complete, etc. the extreme efforts that 5e took to make death of a PC into what is now generally an implausible risk are absolutely impossible to ignore in a discussion of "horror" because they combine to eliminate what makeup the many survival elements 5e designs against with the survival horror elements d&d previously did an ok job handling ok enough to claim that the system provided mechanical support for.PC death is quite plausible in my games, it's not that hard. Even when you have access to revivify or raise dead (which is not as simple as casting a spell in my game), you can't do that if there is no body. I don't typically go that route because of player preferences but if you think you can't kill PCs in 5e you just aren't trying hard enough.
But more to the point I was trying to make is that while a struggle for survival can be part of a horror game it does not necessarily make the game a horror game. Shipwrecked and stranded on an island and missing spell components or the party simply doesn't have someone that can cast the necessary spells (i.e. no cleric so no create food and water) does not make it a horror game, nor is it required for a horror game.
There are obviously other games that have more mechanisms for a horror game, whether they're effective or not likely relies more on the players than the system.
When the bar for gritty has fallen to that one time Adam West Batman got socked it drags any sort of "horror" with it to where almost any form of minor struggle that cannot simply be a thing the player can choose to ignore without consequence on a whim gets dubbed horror. In the context of modern d&d it's no longer accurate to suggest that it's about targeting anything so much as the world having more weight than Adam West Batman villains because the mechanical elements no longer function in ways that carry any other forms of that weight.The trick to doing horror is to not target the PCs. Irrespective of PC power, fighting big scary monsters is what they came to do, and a regular part of every D&D game. Don't even bother to try. Even if you make the game more deadly, then character death becomes just another joke (and this is true for CoC and the like too. Your disposable character goes insane is just another joke).
That doesn't mean you can't horrify the players though. When the players open a rabbit pie, and realise what happened to the annoying were-rabbit NPC they were all taking the mickey out of earlier. Or when they are exploring an abandoned children's home, and discover a rocking horse with a large rusty iron spike protruding from the saddle. etc.
Death saves, bonus action healing word bonus action lay on hands bonus action healing light, overly generous recovery mechanics with a near impossible to fail at bar to complete, etc. the extreme efforts that 5e took to make death of a PC into what is now generally an implausible risk are absolutely impossible to ignore in a discussion of "horror" because they combine to eliminate what makeup the many survival elements 5e designs against with the survival horror elements d&d previously did an ok job handling ok enough to claim that the system provided mechanical support for.
The problem with that bolded bit is of course the fact that you used a work of fiction that explores heavy duty psychological elements that are entirely inappropriate for a ttrpg & does so in a verygritty post apocalyptic setting. Glenn may have been taken out in S7E1 but comic readers spent like six years knowing without doubt the leadup to it & how it would play out since before S1e1. Heck I asked google what viewer discretion warnings TWD had & it's paywalled but apparently "Intense Psychological Distress: It regularly depicts human-on-human torture, psychological manipulation, and the sudden deaths of both adult and child characters" I recall both the comic & tv series going a good bit above & beyond what might happen at my table.
If you had namedropped romero's night of the living dead or shaun of the dead it might have been a solid point, but those aren't exactly horror so the point looses a lot of steam on the grounds that one is a comedy & the other is more social commentary than horror. Using 28 days/28 weeks/28 years might have worked since there is so much action, but those fast zombies are freaking scary in ways no monster in d&d 5e can be.
TL;DR: Maybe revise that original point so it draws on a series with a "struggle for survival" that wouldn't win top marks on /r/rpghorrorstories all time hall of fame if used at the table? d&d5e is just incapable of carrying TWD's gravity & earlier editions probably couldn't either.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.