I understand your thinking, I lean towards wanting characters to find redemption myself. But let's be clear here, what she did wasn't a mistake. She cheated on her boyfriend for who knows how long, then when she was caught she let the dude beat him then to his face coldly called him a stalker and abuser. She then let Kondo bruise her arm, take a picture, blame it on Eiji, and let defamation and bullying of Eiji spread throughout the school (defamation and bullying that would have led to his suicide), all the while never correcting it or calling it out, and even affirming it at times.
Her choices were going to kill Eiji if it wasn't for him meeting Ai, all the while defaming his name into disgrace in the minds of all the students there. That is not a mistake, that is a culmination of choices done by a malicious and evil person. Regardless of how you feel about the severity of her inevitable uhh spoilers, you have to acknowledge what she did was just as severe against Eiji.