Hi Rob, gave this a read.
Format wise you should open directly after FADE IN.
** A man runs alone in the dark.
EXT. CITY NEIGHBORHOOD - NIGHT
** That description line (A man runs etc.) should be here. Below the Slugline.
Perhaps 'jogs' is the more apt verb, given he's running for his life later.
And, perhaps Suburban Neighbourhood is more apt too. Doesn't appear visually very 'city'-like.
I like your villain's name. Inspired by the firearm?https://www.thefirearmblog.com/blog/2013/12/02/sig-sauers-sigm400-muddy-girl/
I don't think you need Mrs Abernathy. Extra character, extra casting, gun action etc. I don't think she adds much. You need to list her house as a location btw, (if you keep her in). Same with the house with the gargoyles, and the house with the creaking swing - Mini slugs will do, but they are specific points of reference/locations you've written.
I also think you could cut a lot of Steve's dialogue.
I think you're attempting introducing a bit more character with your story i.e., Steve in training as Marathon runner. Not sure all his talk works though.
Sig is a good scary character. I just think the story is a variation (albeit a more gory version) of The Smiling Man short films. Quite a few of them out there, including this one below:https://youtu.be/_u6Tt3PqIfQ
Your FADE OUT. : should be aligned right.
Hey, horror shorts are popular. Good luck with this.