That night...

There was a full moon.

I see. And youkai's power increases under the light of a full moon.

Thanks to my work here, I'm pretty familiar with the lunar cycle.

But does that have any kind of bearing on this case?

At her core, Keine's a kind, gentle-hearted person.

I can't imagine her ever wanting to hurt someone.

Let's say some youkai did go berserk because of the moon and attack the village.

Keine would probably fight to protect the humans living there.

In full hakutaku mode, I think she'd be more than capable of killing her opponent.

Although, if it was something like that, I don't see why she'd feel the need to erase it from history.

And we don't know for sure that this whatever-her-name-is was youkai or human.

Perhaps we can start by narrowing down what kind of youkai get powered up by the moon...

If that's the direction you're going to take...

You're better off asking an expert on the subject.