Tatari is brainwashing people with those necklaces.Note how people who are loyal - or semi-loyal to him - always have those necklaces, while those who rebel don't. Also, note that Violetta doesn't have one until after she sides with him...
Tatari is trying to make zombies.Which is why he lied about those dead bodies.
It's all Tsubaki's fault.Tsubaki became so crazed about the obsession with the Rankings that she brainwashed everyone and then killed them.
October will die in conjunction with the Rankings.Because the Rankings are at the end of October...
The time loop will be ended with only one person left alive.Tatari does't care as long as everyone isn't killed and will end things when it's convenient for him.
Lucy did not write the response note to Kaen's. His killer did.It's seriously out of character for Lucy to neglect meeting with Kaen, and it seems awfully convenient his killer knew exactly where he was. In addition, Kaen noted the improbability of Lucy going back to read Kaen's note, so what are the chances of both this and Lucy breaking character like this happening?
There is more than one murderer.The fact that Kaen killed October somewhat brings up the idea that there could be multiple murderers out there. Which is sort of how death from stupefaction ended, anyway.
Shun is the one responsible for the murders.Kaen's murderer is said to have long thin fingers. Just a few scenes before, Shun is described as having spider-like fingers.
Headmaster Tatari is a WitchAnd Redwood Academy is his Witch barrier.
Kaen did do it.Repeatedly it is said Kaen does not remember doing it. Perhaps he has a Split Personality or some other concept.
They're all in Hell.Tatari lied about wanting to stop the tragedy. It's all for the sake of torture.
The murders will stop when Ceoil makes amends with Kasui.It's more than just a subplot. The murders are caused by either Kasui or Ceoil's repressed consciousness.
Ceoil and Kasui had sex.Under the bleachers.
The survivors of Death from Stupefaction are the ones responsible for all of it.If they aren't the murderers, they're conniving with TATARI to revive everyone in a way or another.
Whoever the killer may be, they're really good with target practice.All murders except for the accidental one Kaen did had the killer to shoot, throw or precisely slash something, many times in a really tight chronology to follow. Whoever is the murderer, they know how to keep their timing and precision. They also have long and thin fingers, which is something a precision striker (like a surgeon or a pianist) tends to have. It's not impossible to have someone like that in Redwoods, as it's a school that gathers talented people from every field. there's a chance the murderer even knows TATARI's secret or remembers all the time loops.
Everything after the first few scenes is all Sozoku's dying dream.