That's the real end. I know, I killed every main character, literally, how could I do that?

Well, how couldn't I? It was all planned from before I even posted the story. Oops.

But hey, G-Dragon, TOP, and Jay Park and Co are still alive.

I feel like I should probably clarify a few things, in case anyone is confused. If I leave any other questions unanswered, just ask, I promise I'll explain as well as I can.

1. Levi, or Angel, did in fact die in the beginning, killed by the Double Dragon at the gas station. I did hint at this. The dreams and visions of Levi dead that Sin had were the repressed memories of Levi's death. That's why there wasn't really any substantial interactions between her and the other members of the gang - no one besides Sin really said her name or awknowledged her (any interactions were only observed by Sin, weaved together by her imagination and brought to life by delusion). This is also why Levi hardly ever spoke. She was often said to feel cold, and her nickname was Angel. Makes sense now, I hope.

2. Sin, or Xan, suffered from a mental break after Levi's death, and deluded herself into thinking that Levi was still alive. She repressed any memory of Levi being dead - however, the feeling of hatred towards the Double Dragon was reflected over because they were strong enough to pass through her memory blocks. She just didn't know that the deep hatred for them was because they killed Levi.

3. Star was the second (Jin died before the story began) of the original gang to die (the gang before Sin joined, and Levi never joined the gang because she died before meeting any of them.) Star was killed by B-Free when he and 2 other Double Dragon members came to the Underground after Suga and Namjoon's championship win. Truly, Star never gave Sin a gun. Sin made this up because she thought if she had had a weapon she would've been able to protect Star. In reality, Star approached the men first, and after they shot her, Sin stole her gun. It was blank because Star had already used all of the bullets. That's also why Sin was so willing to easily pull the trigger at B-Free's head, though she didn't know it because she blocked any acknowledgement of Star dying, much like in Levi's death.
The gang had known Star for a while before she died. That's why there were interactions between Star and other members, and she spoke even after her death, because Sin knew her personally and she was alive at some point in the gang. Though after, Star almost symbolized what Sin wanted to be, and Star's essence embodied Sin's warped sense of justice, revenge, and conscience.

4. How could Sin kill all of the Double Dragon alone? Friendly fire can be a bitch, always. And, delusional Sin didn't really register any of the injuries on her body, which kept her running. Add to that the adrenaline and drugs in her system, the blind fury, and complete disregard to survival, and she was as close to achieving superhuman status as humanly possible. That being said, it's up to you to believe if she alone really did do that much damage.

5. The whole Jin thing?
It's up to you guys what to believe. Think that he really was haunting her and speaking to her from the grave? Damn, you guessed it. Think it was just a figment of her imagination and wasn't the real Jin? Shit, you right. Was it a hallucination from drugs? Damn how'd you know, you got it.
It's up to your interpretation, just like the flowers having special meaning in the story. To me, they do though.

6. When the other members started dying, why did Sin call them by their real names, but continue calling Levi and Annie by their gang names, Angel and Star? Well, they died way before the others, and Sin was so desperate to cling to her denial that she continued calling them the names they had given themselves.

Information:
All members did drugs, specifically Crystal Meth. I didn't want to get deeply into that, as Sin pretended it wasn't a thing, but it's true.

The two mirror scenes:
Near the beginning, Sin stared into the mirror while Namjoon kissed her, and she described the mirror as clear and untarnished.

Later, the situation is the same, but this time Sin sees the mirror as dirty and tainted. This was to signify a change in Sin's thoughts on everything that was happening and how she felt.