(there could be bad grammar...)

So miles when he was a kid was bad treated by his parents (no not like abusive, i meant like they don't care much about him and leave him with himself) basically his father loved watching horror movies, and when his father and his mather went out for shopping, he forget to exit the movie, leading miles to watch it fully, sadly he got obsessed with the new experience about blood, murder, and much violence stuff, he searched for a knife and start practice to use it, he hurt his fingers many times, but luckily his parents comes back and stop it, and miles being a kid was upset about this, months later, miles had practice many times with the knife secretly and he thought about "wait, how it feels to stab someone?" and he tried to stab his parents while he was asleep, leading to their deaths, miles didn't know death is permanent, so he just walked away satisfied of the experience, after few days his dad's phone rings, but miles don't know how to use a phone, so he left it until it stops, after few hours, his uncle decided to enter the house because miles' dad didn't answer, he saw miles being on the living room watching horror movies, the uncle stops him then went to the bedroom seeing both of them dead with bad smell of their bodies, the use called the police, the police tried to find who's the murder, but the only one in the house was miles and miles is like 4 years old, there no way he killed his own parents in such bad way, the uncle decided to pick up miles and make him live with him, it didn't passed a day and the uncle saw miles playing with a knife, he stopped him and tell him how bad playing with a knife could be, miles understand but still upset he can't play with the knife, few years later, miles became 16 at this point, he saw the knife on the kitchen leaving there... Then he remembers about the way he stabbed his parents, he wanted to feel how was the stab, but accidentally stabbed his uncle when he tried to stop him, then miles realize the bad thing he did, his wounded hands is filled with darkness, and from now on, he will never think about using the knife ever again, and there where miles wanted to live a happy life, plus that's why he doesn't be terrified when he sees a dead body (aka when he saw miss circle chasing abbies and then miles saw abbies being killed) and he never fears any kind of violence, but he only backs away because is flashback his trauma

Extra (not part of his backstory, just made it for fun): after miles being known as the smartest kid in math, miss circle decided to do a challenge for the kid, but the problem is she took it too far that the math challenge can't be solved by human without a help from tools yet she doesn't allowed tools, not only that miles notice how hard it's, he also failed miserably, making miss circle disappointed, of course, she now want to kill miles, miles did try to run, but miss circle managed to cut his upper left arm, and miles start getting flashback from his trauma, making him mindless, he grabbed a large (not too large) metal ruler and focusly put it on his half-cutted left arm, it made miles take off blood from his mouth, but he didn't care and start fighting, and miss circle is stronger, he managed to knock him out, but impressed with his fighting skill and the long he stayed, as of no one ever managed to survive from miss circle that long, so with some threats, she made him her teach partner and also killer partner.

If you made it that far, thanks for reading!