My name is Dawn,Dawn Greyson I'm 15 years old if I remember my age right. I'm a pretty normal person I have a good family a good education system and more. Life is filled with ups and downs right? Anyways as I was saying I'm just an ordinary person living a normal life. What do you expect? A serial killer might kill me or turn me into one? Well the second one is a bit right but I don't know. Those vengence counts? Oh before I start that heres what happened to me. I was living with my family my mother,my father and my older brother Peter. I am very close to my brother we both have a good connection. My life in school is great I'm not that popular but I have friends. Oh I'm not an introvert like other people you know what I mean. Let us continue... I have good grades but why did this happened to me? Aren't they happy? Aren't they happy for me? No not them I was talking about the person who did this to me. I was walking home on my own because my brother is sick at the time and our car is broken so I have to walk. Until I spotted a person following me. I brushed it off and continued but he keeps in following me. With out thinking I ran. I sprinted away from him then he ran. He was pretty fast that he caught me. Then I don't remember what happened. I just woke up in a surgical bed. I can't move or speak. Then I heard him mumbling something. Then he sharpen a blade the loud clinging sound of the blade gave chills in me even today. I assume he came near me due to his footsteps. Then he placed the blade right at my stomach. I felt the intense pain I wanted to scream but I can't. I just lay there how stupid of me. He then put a needle that made me fainted was it anistetic? I can't remember, I thought I was dead but I'm not I should be dead right? All I remember was waking up in a pergatory. I can't even move until something triggired me a sound. Aren't they happy today? Aren't you happy today? The voice keeps echoing in my head while I kicked my the small door. And it break I can move again. As I ran I felt something painful in my stomach. I checked it and there I saw stitches around my stomach. Not only my stomach has stitches my whole body has stitches. I quickly saw my reflection to an iron door my face was filled with stitches my left eye was removed which I assume the person performed Enucleation. My mouth was slit ear to ear. Which made me freak out of course. Then I felt somethong metalic in my stomach. I removed the stich and saw what is inside my stomach a huge pair surgical scissors with a needle tide into it. I felt sick that I could vomit. But I felt not myself anymore. I restitched my stomach and stitched my slitted mouth. It was painful but I heard the voice again. Aren't you happy today? I lost myself that I fixed my dirty brown hair which is all around my face. Then I heard a person comming in the room. I tried to run but my body just stood there without thinking I stab the person with the scissors and he bleeded out. What did I do? Did I lill him?! I was scared I grab the person placed him in my pergetory and switched my clothes to his. I wore a white hoodie with black sleeves and some jeans. I also wore his shoes. I ran away from the place. I put my hood up ane begin walking as I calm down. I was the one who killed people in there sleep can you see a messenge? But the real question is Aren't you happy today?
First this is a wall of text. Break it up into paragraph. Whenever a new subject begins you start a new paragraph, whenever a new person speaks you also start a new paragraph.
You also have a lot of confused wording in there, "those" instead of "does" for example. Your grammar is off too, tenses are all over the place. Proof read this please.
As for the plot, it just feels like a typical teen gets insane and kills people in impossible ways. I suggest you look up the stories of real life serial killers who started off young (late teens, early twenties) and look into why and how they did this.
Is that a psychopath or has some other issue that affects empathy or is it something else? If it's the former, thus you'd have to make them seem like a somewhat shitty and/or akward person socially in the emotional sense and maybe display them abusing animals (though careful on the details). If it's the latter, have them have some problem that stems from their environment, like an abusive parent - in this case have them have a certain "trigger" that sets them off into violent bursts that slowly leads up to lethal violence. These triggers could be disrespect that reminds them of their treatment by their abuser, or someone who physically or behaviorally reminds them of their abuser, if they have like similar mannerisms etc. Percieved loss of control over a situation could also be such a trigger, there's a lot of ways to go around it. Just do some research. It would also help if the same main antagonist, your narrator was either exceptional at something (be it extremely charming and manipulative or just smart enough to come up with unique ways to overcome various difficulties that could arise from attempting to murder people) thus making them an efficient enough to compensate for their youth or have them be a massive human being (not 8ft tall though).
Realistic serial killers are pretty alright if done right, usually they aren't though, at the moment you don't have a realistic one. You can use that title catchphrase as a calling card for your OC.