I thought this was very clever. The way you included the grotesque descriptions was very effective in setting the creepy mood. This story could have easily come out sounding silly but you've managed to avoid that by keeping it short and focusing on the actions rather than the interactions between the people.
The only trouble I really see is with your spacing. Usually quotes and new paragraphs are denoted by a double space. I confess that I'm not sure if this is an actual grammatical rule, but it is the form I'm used to seeing so seeing anything else is distracting. I inserted my suggestions in your story.
Best of Luck,
In the afternoon I sat with the book on the deck. I liked to be at the cottage, especially at this time of the year. Late August had advantages: warmer water in Georgian Bay, fewer people on the beaches, deeper shadows in the forest. No space needed here. Would be better as one paragraph.
Spots of sunshine filtering through the branches of the nearby trees flickered on the porch floor. Crickets had a concert in the tall, yellowing grasses. Blackberries ripened slowly to perfection. With my eyes half opened, through the eyelashes I observed Mark unpacking something from the car.
I'd double space here since it's a new paragraph.
Mark had a strong muscular body and a boyish face, despite gray streaks in his hair. I liked everything in him, blue-green eyes, soft voice, a smile and a way he looked at me. Except those two times when his face turned white and his eyes became stainless steel cold, but that was on the beginning, at Christmas, now is Summer, our first Summer together.
Double space to isolate quote.
“Daydreaming?” Mark asked. He appeared suddenly on the deck. I turned, sun shone straight in my eyes. Something heavy fell from Mark’s back, a large bundle covered with black plastic. Mark stood looking down. I noticed shiny pearls of sweat on his forehead and large blue veins on his hands.
Double space for quote
“You have to help me this time. You know what to do.” He stepped in front of me blocking the sun. “We don’t have much time, The Larsens are on their way, I will pick them up from the station in Midland, but first I have to finish something. You know what to do.” Mark turned and walked toward the garage.
I knew what to do. Karen and George will come from Toronto in about two hours. I had to rush. The plastic bag on the floor swished and moved. My heart pounded. “You know what to do.” I knew, I never did it before but I saw on TV. Through the porch doors I went to the kitchen, opened the cupboard and took a hammer from the toolbox. I weighted it in my hand; the yellowish leather handle had oily spots. I took from the sink rubber gloves and a large knife from the stand on the counter. I checked, the knife was sharp, sharp enough. “I know what to do” I repeated to myself and stepped outside again. I looked around – nobody else, only me, crickets, a blue lake and a light wind high in the crowns of the trees. I hunkered down. I slightly uncovered the corner of the plastic bag, placed the hammer in the back of the head and hit.
I didn’t like the wet, squashy sound and a sweet taste in my mouth. I hit second time turning my head away. I jumped out when the body moved. I looked down. The corpse trembled, twisted spasmodically once, twice… one more convulsion and silence. I stood waiting.
Step after step, slowly, quietly I walked back. Ice cold drop slid down my spine. With pumping heart, I pushed the body with a tip of my slipper. It didn’t move this time all covered with plastic, all except the pierced corner.
The look of the bulging, glass like eyes and wide-open mouth brought back the sweet, sticky feeling in my throat. I noticed stir somewhere in the bushes. I froze. A squirrel darted between the trees. I looked around. I was alone. I grabbed the plastic and pulled hard to move the corpse deeper to the shadow.
Combing my wet hair I looked in the mirror. “I am a killer a MURDERER.”
Double space for new paragraph
I had to hold the edge of the sink when I recalled what happened in the last two hours. “Who am I now? I cannot say that Mark pressed me to do it. I just wanted to help, to do my share. The look of his eyes! I didn’t want to see again this coldness and distance. After all MY friends are coming to visit, I HAD to do my share! I had to!”
Double space for new paragraph
“Compose yourself and finish,” I said out loud and went back to the kitchen. I looked around checking again every spot. Kitchen shined. I cleaned everything methodically with the sharp brush and soapy hot water: the hammer, the knife, the electric saw, the cupboards, the counter and the kitchen floor. I scrubbed bits of meat from the tiles above the sink. The strong smell of chlorine brought me a strange relief.
I would include this in the last paragraphI looked in the fridge, large pieces of meat stacked neatly in layers separated by parchment paper. Double space for new paragraph
“Mark will be proud of me, he wouldn’t do it better.” I thought. I put bread on the table, bottle of wine, forks, glasses, napkins. I turned on the radio to kill the silence inside the house. ”No, no they can’t take that away from me” old swing standard sounded just right for the occasion. My hair almost dried after a shower. I looked back at the kitchen. “Perfect” I smiled to myself and returned on the porch. I checked the barbecue. “Clean and tidy, Mark does really good job. Everything ready for my guests. They should be here any minute.”
Crickets had their usual concert, blackberries shined like freshly polished, birds sang their love songs. Aroma of balsamic pine, heated soil and mushrooms filled the air. My act of violence passed unnoticed. Nothing happened. Nothing changed. Undisturbed comfort and peace. I stretched in the chair and looked against the sun at the lake.
Double space for quote and clincher
“I won’t do it again.” I said out loud, my voice sounded unnatural, rough, like not my own. “I will never kill a fish again. Never!” I said louder and sank deeper in the chair.