When a director says they're making a movie "for the fans", odds are that it's not going to be a good movie.
(Note: This isn't anything unique to horror film fandom. This is pretty much the case for anything from sports to music to television to whatever.)
I think a lot of this comes from combined senses of loyalty and protectiveness. Fans identify with their chosen interest, as well as the community surrounding that interest (the fandom itself). The interest is precious (hence the protectiveness), and the community is social support (hence the loyalty).
One byproduct of all of this is a weird defensiveness, as if fandom is responsible for protecting their interest from those who would criticize it. And that leads to some petty-ass bullshit.
Case in point: A brief news item on horror website Bloody Disgusting, titled "Rooney Mara Spits On Her 'A Nightmare On Elm Street' Role."
The upshot is that actor Rooney Mara said in some interview that she didn't especially want her role in the remake of A Nightmare on Elm Street, but she did well enough in the audition to get it, and was all "holy shit, I nailed that." Which, okay, maybe a little goofy as far as stuff to talk about in an interview, but the writer treats it like she burned the flag or something. An excerpt:
Still, she doesn't have to piss all over a film that she clearly used as a career stepping stone; treat it like a battle wound and wear it like a badge of honor. There's nothing I hate more than excuses...Battle wound? Badge of honor? I mean, come on. Like it or not, not everyone looks at a role in a horror movie (let alone a remake of a horror movie) as something to which they aspire. But it's work, and if you want to be a working actor, you have to work. Even if it's not great art, which this particular movie was probably not going to be. Why pretend otherwise? It's not some kind of blow against horror film, it's just one actors' opinion about one movie (which received mediocre reviews and did mediocre box office, and received a 2/5 rating on the very site that published this). It's not news, and it's certainly not worth publishing. Who cares?
Fandom cares. Because it's all personal. Her slight on a movie nobody especially loved is a slight against them and everything they stand for. And that's not the sort of mindset - especially among a very vocal demographic - that is conducive to making good films.