I have a feeling the victim framed Uichi for something horrible, which empowered Eito and his gang to make Uichi's middle school life hell (for hypocrisy and "fun") and got everyone to "want him to die" for some reason. In one timeline Uichi reflects how "once again I was hated and betrayed," with an image of the mysterious boy appearing behind Yae (who was revealed to be his first high school bully). If what Yae did is comparable to the betrayal of the mysterious boy... seems like it was very heinous.