I've read the webtoo and I'm honestly quite disappointed that they didn't follow the ending...the whole reason Harin picked on Ja-Eun was not because 'she used to be a bystander and didn't play with her' but because they were sisters. their parents where poor and couldn't take care of 2 children. the wanted to put one of them out for adoption. the sisters made pact but Ja-Eun betraid Se-eun/Harin at the very last moment. Harin had to leave, she endured a lot of torture in the orphanage. one day she saw her sister with their mom being all happy and she decided to get revenge.