I'll start with the usual disclaimer of: take this with a grain of salt, think about the things I'll mention yourself, and horror is not my expertise (though I wouldn't say anything really is).
Also, if there is something more specific you are curious about, feel free to ask. Specific questions can help with acquiring desired feedback, as otherwise the feedback can end up a bit more general than you might be hoping for.
Oh, I should also mention that I consider writing to be very much a thing where you ought to find what works for you. That's why one of the things mentioned above was to think about the things mentioned yourself, since you might find they are irrelevant in your case.
Technicalities
Punctuation
Because you seem to be punctuating dialogue correctly (here is an article for those who don't know how) I'm going to instead say... that it seems you haven't used any semi-colons or em dashes. Obviously you don't have, but in case this isn't a choice but because you don't know how to use them, here is a website that might help get some idea of it (to start at least, there was a better one but I can't find it): liminalpages.com. Alternatively, you can wait for Gower to make an article here, it'll probably be better.
The reason I bring this up is not because there is anything lacking in what you have written that'd be fixed with this, but rather because having access to all the tools can help more interesting and varied sentences, along with better conveying ideas and creating nicer flow. Personally, I don't really use either of these myself, but I would like to at least be capable.
Flow & Word Omission
This one depends on style and intent, so think about it an extra amount.
Here are two sentences early on:
- He worked at Walmart, talking to people all day but never really talking to anyone.
- He liked to read, but only read non fiction.
Maybe it is just me, but the repetition of talking and then read feel a bit off, especially the second one. Try reading them aloud (or mouthing the words). The first one I think just needs a comma (but I could be wrong); check this article out: Basic Sentence Structure: Additive Sentences. For the second one, I think the second read makes it clunky, so I'd just drop it.
As such I'd suggest something like:
- He worked at Walmart, talking to people all day, but never really talking to anyone.
- He liked to read, but only non-fiction.
I'll quickly mention that one thing said: non-fiction/nonfiction (British/American spelling). However non fiction didn't seem to come up.
I find flow to be something you get better at just by writing/paying attention. It is an interesting element of writing which I am no authority on, but I can still point it out!
Other than that, word omission would also cover not giving pointless information. Sometimes, readers can assume things, and this can also be used to affect pacing. As you need to think more about tension and stuff (which I think matters more in horror) it is definitely worth keeping in mind.
-ly
This'll be short because you seemed to do it well, but I'll say that words that end with -ly being tacked on as additional descriptions for stuff can take away from the writing when overdone (this ties in with word omission). Some people tend to overdo this without realising the negative impact it can have. Anyway, you seemed to do this well, so no complaints from me.
Story
Not much to comment on, as this is just the first page. However there are some general things that are worth mentioning.
Show, Don't Tell
Mara did mention showing rather than telling regarding the protagonist being uninteresting, which is worth thinking about as 'telling' can be bad (but again that is when overdone). Showing AND telling do have their merits, but the general proverb exists because people tend to overdo the telling which can end up rather dull.
In this case, I think both ways can work, and it depends on what you are going for. I imagine you don't care to spend much time before the inciting incident, which makes sense, which is why I think keeping it as is should be fine (but do consider trying to polish the opening some more, as you generally want to make sure the reader isn't put off before the story even really started).
Plot
I'm curious about all the directions this can go, you already mentioned a major branch at the start, so it is nice to see you thinking about it. Anyway, suffice to say the idea seems to have potential (but this doesn't mean much, as I'd say most ideas have potential).
However I'd say this opening is executed pretty well, and if you end up delivering some happenings/narrative beats, I'm sure this can end up as a pretty good story.
Scope
Don't forget that the scope of branching narratives can explode like crazy, so thinking about it a bit is a good idea. Also keep in mind that cutting things or restructuring them is something that exists and can be done.
Conclusion
Keep writing, it's looking good! Do tell me if there is anything else you'd like mentioned, for I tried to not go overboard by giving all the general advice I can think of (aha), but this does mean I might've missed something worth talking about.
Oh, also, if you have time, remember to proofread. I think people like to skip that step, but it is definitely worthwhile as it can push a good story into a great one. Doing a proofreading pass at the end can also help spot scenes that might be omitted (or find good spots to add new ones), because at this stage you'll have a really good idea of the overall story.
Heck, in general, the micro vs macro of writing is interesting, and something I'll hopefully get a better understanding of. I was considering trying to talk about it here, but I'll save it for when I am more confident in what I have to say, and besides, this post is long enough as is.
P.S. Here is an article about proofreading (for those who it may concern...which might be a good deal of people): Proofreading.