What's in the medical report is previous outbreak of "fiends" explained, in which actually are the villagers themselves. The only one who can protect the village, granting their normal life, is the miko ritual. Within the village barrier, under the protection of miko, villagers won't turn into fiends. If they were to go out of the border, they'll turn into one as it is out of the miko's protection.
The miko could be anyone, but it'll be easier if miko doesn't have relatives to avoid the trouble of convincing them and/or to prevent the possibility of the relatives thwarting the ritual. That's why Atsushi was chosen. But the current miko, Souma, learnt the true nature of fiends, so he volunteered as the next miko to give Atsushi a normal life to live.
Atsushi, didn't know about the true nature of the fiends, only fixated in making their old promise comes true, feeling robbed off of his master who treated him dearly. He tainted the miko, Souma, leads to the turning of the villagers into fiends.
It's only my suspicion, but I guess villagers that are turned into fiends held negative doubtful unstable feelings. As it was shown, megumi with broken heart and a grandfather to take care is the first to turn into fiend. And then Atsushi's friend, maybe riddled with fear. And then Atsushi himself, in his wrath at the "God", the frustration of finding out the true nature of the fiend
What's in the medical report is previous outbreak of "fiends" explained, in which actually are the villagers themselves. The only one who can protect the village, granting their normal life, is the miko ritual. Within the village barrier, under the protection of miko, villagers won't turn into fiends. If they were to go out of the border, they'll turn into one as it is out of the miko's protection.
The miko could be anyone, but it'll be easier if miko doesn't have relatives to avoid the trouble of convincing them and/or to prevent the possibility of the relatives thwarting the ritual. That's why Atsushi was chosen. But the current miko, Souma, learnt the true nature of fiends, so he volunteered as the next miko to give Atsushi a normal life to live.
Atsushi, didn't know about the true nature of the fiends, only fixated in making their old promise comes true, feeling robbed off of his master who treated him dearly. He tainted the miko, Souma, leads to the turning of the villagers into fiends.
It's only my suspicion, but I guess villagers that are turned into fiends held negative doubtful unstable feelings. As it was shown, megumi with broken heart and a grandfather to take care is the first to turn into fiend. And then Atsushi's friend, maybe riddled with fear. And then Atsushi himself, in his wrath at the "God", the frustration of finding out the true nature of the fiend