Based from everything we acquired from Kyoukai-centric chapters, I could tell that Kyourei wasn't really evil, she's just....a misunderstood soul. Just like what Kyoukai revealed on previous chapter, Kyourei became insane after ritual battle against her childhood friend Shiki( which ends with Shiki dead). I have some theories about Kyourei:
- Kyourei refused to fight, let alone kill Shiki on ritual battle. The latter was like her own sister, they're really close. I was like, "How the barnacle you have to kill your own loved ones, unless they're pure evil?" But the elders forced her to do so, otherwise face the greater consequences.
- As seen on flashback, we learned that Kyourei won't let anybody taught her except Shiki and old lady( the tribe's elder maybe?). I think Kyourei has trust issues here, probably she was abandoned by her own family then was taken care by Shiki's family I guess.