
i have an idea too. what if those girls were his sister's bullies? since it's unlikely for young ladies to be criminals (like criminals-criminals). my best guess is that they're a circle which gave the sister a hard time in the household and society. since she was adopted, she should've been present at some events to save the reputation of the adoptive parents.
but who knows, let's wait for further updates (〜 ̄△ ̄)〜

Spoiler
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No it turned out he had this weird hero complex; he would search for girls in abusive families and kill them one by one in order to "save them". He thought he was doing good deeds by killing them all but he was doing more worse than good since they all wanted a future for their selves.
Suddenly, I am ready to turn a blind eye on this one if those ghosts were actually criminals or the ones responsible for his sister's death.
ps. I am more interested on this backstory than that red head loool