I can only accept this as, there are real people like miyuki people may think that there cant be that much of a idiot surviving in this world but sadly there are. and some still get happy happy endings even after all they done (and in today's world that is much more possible) and exactly thats...
for this story all i can say is, ''started so well, fumbled so hard'' bc it took so much time to even think that he loves her or the other way around. thats why it started to feel like too unreal.
Like u said her sin isnt murder, BC ai was at the rooftop if she wasnt. She would be guilty with assisting murder. That is the reason noone cares about miyuki.
she got lured in by an experienced guy who lovebombed her and stole her kiss etc. but i think the cheating part started from when she first wanted kondo's attention. even if she is young this just shows that she had it in her bc she had a lot of chances to tell eiji that she want to break up, if...
kondo is the literal satan he is pure evil and enjoys breaking lives of poeple, i dont think miyuki is worse than him but the only reason i cant accept miyuki is the one she betrayed wasnt some random dude she met recently if it was like that she could of saved herself with what she has done...
the more she sink in, the more its not possible to do the right thing. at first she tried to do the right thing multiple times, kondo didnt let her at first, then her being scared of getting backlash of lies at school kept her from doing the right thing, and then when she realized she betrayed...
as sensei said they r kids and dont understand that the only ones fuckedup are themselves, they r victims too and she is one of the biggest victims of the story. author tries to emphatize that after in the story but i cant forgive her just bc he was about to die at the rooftop she choose her...