Rise and Fall/WMG

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...

It's seriously out of character for
 * has ample access to paper, so it's very likely
 * They're at a school. There's a lot of paper.

There is more than one murderer.
The fact that somewhat brings up the idea that there could be multiple murderers out there.
 * Addendum: It's a chain reaction of killings. Each killer is trying to get revenge for the last murder; for instance whoever killed in supposed revenge for the shooting, thinking that  did it.

Shun is the one responsible for the murders.
murderer is said to have long thin fingers. Just a few scenes before, Shun is described as having spider-like fingers.

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.
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.

Whoever the killer may be, they're really good with target practice.
All murders 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.