im guessing white hair kicked black hair off the balcony when they were younger (to exorcise the ghost) but forgot kicking a person (off a balcony) will injure them.....im guessing
Rather, I think he was too late to help his friend causing the friend to fall even before he reached the top Or he dismiss all the sign that the friend was possessed, leading to the friend injury
im guessing white hair kicked black hair off the balcony when they were younger (to exorcise the ghost) but forgot kicking a person (off a balcony) will injure them.....im guessing