Uber spoiler about the ending here.
Spoiler:
Yeah, you're right about her being the one that killed them all along.
The "boyfriend" learns of the truth from Akasaka and Oishi and confronts her about it. She denies it at first, then she remembers about it.
: "Satou-kun you must hate me don't you?!"
:
: "But I love you so much!" *stabs*
: "Sure you killed your parents and made it so that it looks like your mum did it, but I love you so I'll always be there for you!" *faint*
... I don't have the book with me right now so I can't guarantee 100% accuracy on the dialogue there.
