You should post a sampling of your theories for posterity's sake.
Alright. I feel like mentioning who is alive and when spoils the game for people earlier in than you. i.e. "If X is alive for Chapter 4, they must not be murdered or convicted in any chapter prior." So I'll spoiler tag all of this.
I feel like there's zero chance it's Hiro. The game is playing him up as prime suspect, so it's basically guaranteed. I think Celeste is a good possibility as the culprit here seems to be very calculated. I have a theory Toko/GJ might the spy, if nothing else. That's pretty much where I'm at, mentally. I don't think it's Kirigiri. I think of she's going to let Makoto down like that, it's not going to happen until the last possible opportunity, but in general, I believe her when she says she won't kill anyone."