See, now you have to prove that a murder of 100 people is a part of some preparation or a ritual instead of a by-product of some disaster.
Case in point, when 12 people got missing in 2012, there wasn't a mount of corpses to complement it. Because that experiment was actually planned, rather than hijacked.
Exactlly, the name of the disaster is Seiji.
I think that this is the most likely scenario:
From what we can guess and what Kaiano said to Seiji the Experiment is similar to the Juunimon ritual, it involves stand-ins to better control it, said stand-ins are tricked into thinking that they are not sacrifices while it seems that all the twelve people involved are in fact ones, the only exception being the thirteenth among them.
Seiji prepared his grand return from the dead, probably through Sawada, using something similar to the Juunimon ritual to hijack the Experiment.
Now there are two possibilities:
1)
This involved killing dozens of people, probably both to hijack the experiment and disrupt Juuzo's plan, this is why the head experiment, Taketatsu and the prospect maiden were chosen.
The mass murder was just the set-up for Seiji own version of the Juunimon ritual, thus seems that at least one sleeping agent is involved, Sawada, who most likely organized the mass murder as well at the behest of Seiji.
Tokigawa willingly partecipated in the preparations, killings, that he is a sleeping agent is unknown.
2)
By interfering with the experiment using Sawada as an agent Seiji unleashed the Gosts' malice, by consequence everyone got influenced in some way and the killing started.
So Tokigawa's revenge was a consequence of him being influenced by the malice instead that being part of a planned set-up.
Somerthing didn't work as planned, obviously, less obvious is what go wrong, see the fact that we possessed Seiji's body, others seems to have done the same.
Juuzo tried to regain control of the whole affair by trying to rescue Mitsuki and turning Uehara against Seiji.
Now we are going at the roots of this whole mess it seems.