tbh this isn't only a s korean thing. if you look at how teens this day and age use social media bullying has always been a problem. and if you have been a k-pop fan or k-drama fan or just into korean pop culture, etc you understand that school bullying has always been a big problem in s korea. that's why the whole school/workplace bullying is always a thing in k-dramas.
as for just idols being "outed" as bullys. it's probably not that much of a stretch to think that some idols have at one point been a bully. but i think when they choose to be a trainee if their company chooses to debut them they probably find a way to hide the idols bullying past or pay off victims to not out them. i agree with one of the people before me that said the victim probably was never listened to before and is really upset seeing their bully that made their life hell get successful so now they have a way to get "revenge."