I decided to post this as a new chapter to make the answer easier to find.
So here goes.
I realized that Yamato is always the killer in these things, so I decided to follow tradition and make Yama Ishada the killer!
(I figured people wouldn't think about him since it wasn't
original. By the reviews, only one person agreed… who ever ~_^ is)
It wasn't Taichi because he would never think like that of Hikari, also, the reason Hikari was crying in the beginning is obvious, Taichi was the closes person to her. (In the Digidestined) since both Daisuke and Takeru are alive, it seemed obvious that she was crying 'cause Tai was dead.
It wasn't Ken, Sora, Koushiro, Daisuke, Takeru, Miyako, Iori, or Hikari because they were in the actual fic.
It wasn't Jyou because… "Ken looked at me with hatred and understanding mixed into one as Koushiro's body fell. Funny, the look seemed familiar, maybe all raven-haired boys gave that look before dieing" Jyou and Ken have the same shade of hair, and it said "before they die" meaning Jyou was about to die when he gave that look…meaning he's not alive any more.
So it could have been Mimi…and any one who guessed it was…RIGHT!!!! (congratulations!) Although…the fact that the person knew Hikari well enough to say that she's a "player and a bitch" tells me that it wasn't Mimi, since she's in America and hardly sees Hikari at all, and it's unlikely that she came to think of Hikari as a bitch when they knew each other better, (and Hikari was only eight)
Who ever said Yamato… congratulations!!! YOU'R RIGHT!!
Yamato said he killed Takeru not because it was a joke (I don't think he would be that cruel) but because he wanted to make Ken into a sobbing heap so that he wouldn't try to stop what Yamato was about to do. I thought it was a good idea, to make the story shorter and more to the point, without letting Ken do what he usually does…coax Yamato into letting the gun fall and going to the police peacefully.
I think this might have been based on too much assuming and not enough facts, and for that I apologize, I know that people think differently and not everyone would assume the same thing, if you did NOT get the right answer, don't worry, this was probably the worse "mystery murderer" fic on FF.Net and beyond. ; )