HALF-DROWNED HOUND 
DID and Horror:
Warning: This article contains spoilers for the movie Malignant.
It took me a year and a half to get around to watching James Wan's Giallo-esque Malignant, but it was well worth the wait. Seriously, I loved it, campy bits and all. The score is a masterpiece, the blend of CGI and practical effects is exemplary, and the villainous Gabriel May is honestly sorta hot if you're a bit deranged.
It was also excruciatingly uncomfortable to watch.
Well, yeah, it's a horror movie, so that's the point- but it wasn't for the reasons Wan and his fellow creators were hoping. The reason Malignant sets me on edge is because I couldn't help but think, Are they talking about me?
Don't worry, I don't have an evil parasitic twin who was shoved into the back of my skull and now takes over my body to commit atrocities that it makes me watch. What I do have, however, are dissociative identities, and in the world of horror, the two are one and the same.
Psychological horror has long been entrenched with ableism against people with dissociative identities, from Psycho and its proto-slasher Norman Bates' "Norma" to Perfect Blue's Rumi Hidaka and her murderous pop star alter ego to Split's Kevin Wendell Crumb and his cannibalistic alter, "the Beast". The message comes across loud and clear: "Something is going to use your body for evil, and the audience is going to eat it up."
Even when the characters are shown to have had traumatic childhoods- a necessity for the development of DID- and are given a degree of sympathy from the narrative, they are, in the end, still reduced to monsters. In a way, this is worse: it's acknowledged that the disorder is caused by years of mental agony, but we're not humanized through our pain; instead, it's become the darker and edgier equivalent of falling into a vat of radioactive muck- "It's too bad it happened," the audience will say, "but they're still a villain."
Now, Malignant isn't an ableist movie, at least in my opinion. Between Gabriel having originally had his own body (sorta), the indirect communication between him and Madison, and Madison's lack of amnesia, it's safe enough to assume that it won't conjure DID to mind in most audiences, or at least I hope so. I'm using Malignant as an example not because of any explicit bigotry but because it demonstrates the way being a horror fan with dissociative identities can be a horror movie in itself.
Even in movies that don't rely on ableist tropes for their entire plots, I've been jumpscared by the mention of my disorder as it's thrown into a conversation to heighten the tension. Take, for example, The Taking of Deborah Logan, wherein a doctor suggests that the titular possessee's eerie, unremembered nighttime behavior isn't just not paranormal, but also not related to her concurrent Alzheimer's, and that it's actually "multiple personality disorder". (How quaint.) [ADD MORE HERE]
While it's not a charged statement when stripped of context, the effect on the dissociative viewer, at least in my experience, isn't a rise in anxiety for the mystery to finally be solved, but a reminder that, even when they're not actively demonized- after all, the prospect is rightfully dismissed by the protagonists- dissociative identities are treated only as narrative devices, and sloppily handled ones, at that. DID- which, by the way, hasn't been called "multiple personality disorder" since 1994, 20 years before release of The Taking- is, to the detriment of those who have it, under-diagnosed, with those who actually do get diagnosed spending an average of 7+ years in the psychiatric system before anyone figures it out. The inaccuracies may be brushed off as a simple irritant by most, but the message it sends to those affected by it is that our reality doesn't matter and that our way of being human is first and foremost a literary tool (see more specific gripes here. I care a lot about saneism.) We weren't factored in as potential viewers; we are, to these and many other writers, abstractions.
The isolating [ADD MORE HERE]
(Final note: While there are certainly other pressing issues with many of the movies I've referenced- the transmisogyny in Psycho; the racism in The Taking of Deborah Logan- I chose not to include them in the main text. This is partially because I wanted to streamline the flow of the article, but mostly because I'm not qualified to speak much about either of those. However, I wanted to make it clear that ableism isn't the only problem I have with these movies- just the one I'm focusing on here.)