Some things I find suspicious about Koin.
First is, of course, Koin ending up being Hatami's personal doxxer. Every shady info we find out about him comes from Koin: his talent, the slaughter, his name situation...
Second is the fact that Koin is indirectly responsible for Miharu's death. He's the one who reveals Hatami's talent to everyone which makes Seki connect the dots about the latter's involvement in the slaughter of Adagura film club. That leads Seki to uttering some unpleasant words about Hatami, which ends up being recorded in the Hate Box. Miharu hears that recording, assumes it to be Hatami's, and goes to, ironically, Seki for a plan to neutralise such a threat. Which kickstarts Seki's whole plan of having Hatami kill Miharu. I'm sure Seki would have started scheming something even without the Hate Box, but the reason he realised Hatami's identity in the first place was because of Koin.
Speaking of the Hate Box... Hadn't everyone originally decided not to press it? You know, becasue they were aware that thing was meant to have them kill each other? So how did it end up being pressed? The answer is Hatoyama, who for no reason whatsoever activated the Hate Box. Now, with my favourites, I'm usually ready to stand by all their rights and wrongs. This shit, however, I've got nothing to defend with! He was fully aware that the Box was a motive! And we know he is not as dumb as he pretends to be. If even Arumada could figure out that activating the motive was a bad idea, then Koin definetely can! So if you want to add a fourth (or would it be fifth?) "culprit" to Miharu's murder, it would be Koin.
Of course, putting the blame solely on Hatoyama is wrong. Miharu and Seki are reaponsible for their own actions, and anyone else might have pressed the button out of curiosity if Koin hadn't. ...Or they might have not. The fact remains that it was Koin who activated the Box despite his classmates telling him not to.
There seems to be an ongoing theme with Koin not hearing "no" as an answer. The most evident it is in chapter 3 with an ongoing gag of Koin terrorising Hatami to become his assistant. While I did find it a bit funny, I had an weird feeling about it. I did touch a bit on it in my Matsuoka post, thinking it had cleared up yhings for me. And yet, the feeling persisted. If you think about it, why *was* Koin chasing Hatami all this time, despite Hatami refusing each time? Koin, I believe, is quite emotionally intelligent, and while he has this mischievous air about him, unlike, for example, Kokichi from Danganronpa V3, he is not malicious about it. So why would he keep harassing Hatami, when it's clear as day that he has no desire in participating? Was he that desperate for an assisting student? I'm sure Hishimeki would have eventually given up and agreed. Which is kinda what she did in the end, volunteering to be an assistant after Hatami stormed off mid-show. Koin's behaviour doesn't make much sense to me. It seems as if he *needed* Hatami on stage for whatever reason. Especially considering how he basically forced Hatami to participate, claiming to have entered his name without his consent.
And that turned out to be a lie anyway! He wasn't able to enter Hatami's name, because, apparently, Hatami's name isn't even Tachihaki Hatami! ...As Koin claims. Actually, what's up with that scene before the trial? Koin learns that Hatami lied about his name and unsteady of revealing it to everyone as he always does, he summons Hatami to his room to tell him in private. To just tell him that he knows. No questions asked. No basic "What is your actual name?" or "Why did you lie about your name?", etc. It honestly looked to me like like a setup for a future blackmail or something. But why? Did Hatoyama just decide to be a dick for ruining his show? I can't think up of any other reason for now. This seems to become important later in the trial, since Hatami swears that he did not lie about his name, so there must be some problem with the festival entry sheet. And seeing as "Kokona is actually Coco" theory might be coming true after all, it could become a clue as to how "Hashizawa" could enter her name if she, apparently, never existed. Anyway, that doesn't explain Koin's strange, and almost threatening behaviour.