Hi Mikkel, I've played with the first part. Not sure how much my suggested changes are about English and how much they are irrelevant views on style:

Susan was in every respect, except one, a completely normal teenager. Her aunt Vivian was a little worried that she might become a little nutty had it in her to become a little too nutty at some point. But it was probably just a phase. There was no cause for alarm at this stage. As long as everyone pretended everything was all right, things would most likely be so.

What set Susan  her apart was an her intense love for assault rifles, a love that which she gladly shared with her friends peers. It was her favourite topic of conversation topic. Her friends peers, that is, Lucy and Jane, always tried to steer the conversation in another direction, for when it came to guns, they were not as passionate as she was, when it came to guns. “Guns and Roses” were alright all right, but “One Direction” were better. [Whether you refer to bands in the singular or the plural is a stylistic choice, not strictly a question of grammar]

Susan loved them at a distance, assault rifles, that is, The same way Lucy and Jane loved “One Direction” at a distance, Susan loved assault rifles. She had all the editions of Bazooka! from 1971 - 2011. The glossy pictures with the shining firearms were so real, almost tangible, yet they were something far away and unattainable.

Her parents were so God damn prim and proper. They wouldn’t let her have a gun any! She was only thirteen. And in the UK where she’d had the misfortune of being born, no one, even adults, could buy a real guns. She would have to wait for an eternity until she could get one. She wished that she had been born and bred in the US and not in the UK. 

Susan

You didn’t cut off their noses either, or their knees. You didn’t. It was nonsense. And anyway, what would the military do if they had no guns? They would look silly, like ducks with no bills. Would they take away their uniforms also? They wouldn’t look very frightening. No one would take them seriously. All the school teachers would probably make them look like her father (I don't understand this sentence. I understand her parents are teachers, but why are you talking about "all the school teachers"? Is "teachers" relevant here? If you you might want to rewrite this part). He would slop about at home in slippers and sweatpants. Imagine soldiers like that. It made her shudder. Yes, she got it, there wasn’t supposed to be any army at all. We would just all be friends and slop about in sweatpants. She had, incidentally, completely in accord with her parents beliefs, named her dog “Peace”. What they didn’t get was that in her mind she spelled it “Piece”. “Not every man knows his way around a shooting piece, but firearms are a manly art.” She would write that quote in her diary every day in her meticulous longhand, in pink letters, with the dots over the i’s shaped as hearts. Her tongue would be sticking out her mouth slightly to the side while she was being meticulous. The quote filled her with a shivering sense of awe, and she was proud of herself that her hand was completely steady, while her heart was pounding with like the blows of a heavyweight. It made her think of the steady hand of a sniper patiently zooming in on some fucktard wanker, ready to completely annihilate his stupid skull. She envisaged the bloody goof's brain pour out like diarrhea from a punctured egg. The thought almost made her heart stand still in a timeless ecstasy

MY GOODNESS! This is amazing! There are a couple slightly awkward parts, particularly your use of commas, but honestly, your style blows most native speakers' out of the water. I laughed out loud when I read "fucktard wanker" - such a teenage way of a speaking, exactly as you intended it. Really well done.

Sounds like a great start to a great story.

Just a tip: if you're writing a very long notebook, you might want to post it in several parts. You're more likely to attract corrections that way.