Hello, this is a one-ish page long horror story that I wrote in my Creative Writing class. Hopefully you enjoy:)

Prompt used for inspiration: A young man becomes obsessed with an old man living opposite his building. The young man is convinced that the old man is the embodiment of the devil, and starts planning the murder.

Obsessed

Lately there has been an itch I cannot scratch. I notice this as I watch my neighbor, a retired surgeon, pull his car into the driveway late at night. He’s a strange person. Comes home late, leaves home in the dark each morning, paces back and forth in front of his windows at odd hours. I settle myself in my chair, and bring the binoculars to my eyes just in time to see the old man shutting the front door. I know the sequence of what will happen next. The dining room light will turn on, then the living room light and T.V. The old man will settle himself down on the worn couch, flip through channels before deciding on the true crime channel. He will watch for an hour on the dot before heading up the creaky stairs to his room where he will pace and ponder. After pacing, the old man will check the clock on his wall, before climbing into his bed and flicking off the lights. I know his routine by heart. It has been three months since I started to watch him. Waiting up into the night for him to return home from his outing. 
There is something off about him, but I can’t quite put my finger on what it is. Maybe it’s because I’ve never seen him in the light of day. Only lit up by the orange glow of the street lights. Maybe it’s because I have an overwhelming curiosity. Curiosity in finding out the strange thing about him.
Though I do not know how I got to this point in my life, I found myself breaking the window on the side of the old man’s house in the mid morning light of the new day. Nobody saw me as I pulled myself through the broken glass, fresh cuts marking my skin and dripping blood on the white carpeted floor. I look around the room. I had not expected his house to be so clean. I don’t linger in the kitchen and instead head straight for the steps. The wood creaks under my weight as I climb towards his room. There must be something in there to scratch the itch I have been feeling. Something to point fingers at his strangeness. As I make it to the top of the stairs, I stand in front of his door. Taking a deep breath, I turn the cold brass knob and push open the door. 
“What in the world…” I mumble to myself in confusion.
Every inch of the walls are covered in papers, diagrams, and otherwise gruesome images. Out of all the things I expected to find, this was not one of them. I walk across the floor, careful to avoid the pictures that are arranged against the backdrop of the pristine white carpet. Each image depicts a different horror. Each worse than the next. Human hearts, kidneys, lungs, torn apart, bloody. Dead bodies lying on white sheets, their chest cavity opened and held open with the use of clamps. That’s when I come to the realization that he must be the embodiment of the devil. I must kill him. I look around the room for a weapon, settling on the green lamp on his nightstand. I quickly hide myself under his bed, clutching the lamp to myself, closing my eyes against the horrible images as I wait for the old man to arrive home. 
After waiting for what seemed like an eternity, I hear the old man enter his house and notice the broken window. He mumbles something to himself before I hear the creaking of the stairs. He takes his time as he climbs the stairs. The creaking gets louder as he nears the door. I hear the knob twist, the door swings open and I can see the old man’s polished shoes from my vantage point. I grip the lamp tighter, knuckles turning white. The old man turns towards his picture filled wall, giving me the opening I needed to crawl from under the bed. Testing the weight of the lamp in my hands, I swing it into the back of the old man’s head. He screams in pain before crumpling to the floor, both hands clutching at his bleeding head. I stand over him, the lamp raised to strike again.
“Why?” He asks before I can silence him.
“Because you are a monster,” I say, bringing the lamp down once more and silencing the old man forever. 
I settle on the floor, looking at his limp body beside me. I toss the lamp across the room, my hands, face and clothing have splatters of his blood. A spot of thick, warm blood is forming under his head and seeping into my knees as I kneel beside him. It wasn’t hard to kill him. Maybe that is what is bothering me as I kneel in the freshly spilled blood. Though the old man’s body lies beside me, growing cold at a rapid pace, I can feel his cursed presence haunting me. I hear the sirens in the distance and I listen as they get closer. The neighbors must have heard the scream. It’s no surprise when the police burst into the house, ready to arrest me. 
“Why did you kill him?” asks one of the police officers, putting the cold metal cuffs around my wrists. 
“Because he’s the devil,” I say, eyes glued to what seems to be an empty spot on the wall.
I can see him though. He’s still watching me. There in the corner. He’s smiling. Laughing. At me.