What I understand is that, in writing especially so, when you want to scare the reader, you have to give them a scary atmosphere on the scene.
As for how exactly to do that, I honestly don't know. Quoting Eminem, "I don't know how to make songs like that, I don't know what words to use"
Especially so when the protagonist has means to defend themselves, like a gun.
Well if you want to have a scary atmosphere you always have the basics of the basics: Gore.
Describing bloody scenes with details will bring out a more dark environment.
You can also go build up curiosity (like leaving several cliffhangers on early chapters regarding this situation) and then on the climax of the story a lot of tension. (In this case, when the MC is running away from someone or trying to survive, etc).
Showing the "real side" of Humans is also a good way to make the story spookier. By this, what I mean is that different people will have different approaches in life-threatening situations so creating a new antagonist or villain is always interesting.
When writing a scary scene it is very important to describe the environment around the MC since those details are what will provide the reader that "tension". Of course, it is important to not write a lot of details straight away since, that way, you are delivering too much information to the reader, making him frustrated and sometimes confused. I would advise you to describe things as there is some dialogue or some monologue inside the MC's head. Let me give you an example:
"Please stop!" Danny hears quick footsteps along the dark halls. His eyes wandered around but no one was seen.
'Am I imagining things...?' Soon, the small candles that were emiting some light become one with the darkness. The sound of silence was infuriating but then, he felt the heavy breathing of someone standing beside him.
A shiver ran down his spine as his body froze. He knew he needed to move, he needed to escape this god-forsaken place but his body limits were close: his legs were extremely sore while his head was pounding louder than his own heart.
"Danny..." his voice was rough and as Danny felt sweat drip from his pores he sensed that figure growing closer. Without a second thought he runs foward however, he trips.
"What the...?" suddenly the candles warm light returns and his eyes widen is shock. His arms were trembling fiercely while he contained his gag reflex, "N-No way..."
(...)
Personally, I feel like this way is a lot creepier and you can keep the attention of the reader instead of this:
As he walked down the dark halls, Danny heard quick footsteps. His eyes wandered around but couldn't trace a soul. He didn't know what was happening, how he had gotten himself in this situation in the first place. Heck, he didn't even know if all of this was nothing more than a project from his own imagination.
"Please stop!" he yells loudly.
A shiver ran down his spine as his body froze right after he knew there was someone standing beside him. He could feel his heavy breathing, the one you usually get for running for several hours in a row. His breathing and Danny's pounding heart were the only things that echoed in the darkness of those halls. The candles that were lighten up slightly were nothing more than a memory, since now, they had become one with the pitch darkness along these old halls. The more he felt the warmth of that thing's breath he knew he needed to leave, to escape this god-forsaken place but, just like all his unlucky life, he was aware of the exhaustion coming over his body: his muscles were completely sore, making every movement he made a sacrifice; his head was being pounded by a hammer, yelling louder than his own heart; his lungs were so heavy he even struggled to breathe; his throat was dry like a desert, and his lack of fluids made his sense of direction turn upside down.
"Danny..." something said in his rough voice.
(...)
I feel like as a reader, it is easier to stay in the mood if you use these types of details during dialogue/shorter sentences instead of just a big chunky text filled with details and then dialogue "separately".
One thing that usually helps me write "creepy" scenes is usually hearing dark music. Something similar to
Control by Halsey. It usually gets me in the mood and it becomes easier to write.
Hope I was able to help! Good luck with your story
