OK, now I remember what's going on. I think it's a very weird place to have a flashback, kind of interrupted the flow of the story.
So Yu Juran had to bully someone else to cement her feeling and understanding that she herself had changed and would no longer be a victim. I don't agree with it, but it makes sense as a motivation, and is more sympathetic than some motivations, depending on your perspective.
I finally realised that the other two silhouetted characters are the other two bullies. I'd completely forgotten that there were four bullies.