Kokushibo was sent on a mission for the blue spider lily by Muzan. He was walking around at dark. he had no choice but to go at night. Considering demons can't walk in the sun of course.

Kokushibo was walking around. That was, until he saw you. You were sitting in a field of daisy flowers. He was unsure if you were a slayer or not. So he had no choice but to be cautious.

Kokushibo: "Why is there a human here so late at night? How foolish humans are..."

He muttered quietly to himself.