Oh dear. That was unexpected.
Also, it's weird, but I'm starting to think Celes might be pulling the same gamble I attempted in the forum game (acting as an accomplice to gain an advantage, without taking any substantial risks). Obviously, there's no strike system there, and if the true culprit doesn't get executed, they all die - so the game's a bit different. I wouldn't put it past her, though. And it would make more sense than her incredibly scummy behaviour!
I don't think Kirigiri did it (for meta reasons: I don't see her dying before the endgame). Naegi obviously didn't do it. Syo is undoubtedly clear. Sakura seems pretty clean to me (and I don't see her killing or being killed by anyone other than Asahina). Which would leave:
> Hagakure
> Togami
> Asahina
> Celes
If there's only one murderer, it *has* to be Celes. The whole thing ran like clockwork - the locations, the time of discovery, etc. Celes was the one who orchestrated the whole search for 'Justice Robo'. Her story about Yamada being abducted is all but confirmed to be fake. Besides: it's stupid to murder two people --> twice the risk for the same reward... unless the survivor would be able to expose you. Which would leave Celes as the only possibility, as she was the only character all but shown to be scheming with Yamada. [No one having any kind of connection with Ishimaru.]
If there are two murderers, I'm banking on Yamada having killed Ishimaru (or vice-versa, but I think the former is more likely), and Hagakure, Togami or Asahina having killed Yamada. Maybe. It would fit because for once, Celes seems genuinely surprised (or dismayed) that Yamada was killed. The reaction she had seems pretty genuine, although it is telling ("I couldn't have foreseen it" = "Derp, my whole plan is crumbling right now").
The only thing that bothers me is that hammer. Only someone in the Celes/Yamada conspiracy could have gotten their hands on one of these.
--
When you have nothing to say, quote yourself ~ Solfadore