Well so if I understood correctly the revenge was because her best friend betrayed her and her lover kind of cheated…
I agree with you, people keep saying they both betrayed her. I understand that to an extent but i dont think that was what fueled so yul, the way i watched the movie was that so yul some way felt she was better than yeon hee, it was unconsciously ingrained into her. So she couldnt stand yeon hee achieving what she meant was hers in the first place this is why i think the "revenge" went so far. She wanted to destroy both of them.