I don't understand the confusion/dislike/Bibi is OOC comments in this and the previous chapter.
It has already been stated that Bibi just wants to have someone to love, and she thinks Peter would be that person. During the date arc, she has this fairy-tale-like imagination about her date with him, but it turns out not to be not as she expects. Despite that, she admits she has fun with the trivial things on the date (which, at this time she still does not realize it is thanks to Amy). This killing-Peter arc is a perfect parallel to the date arc as now she questions deeper into why she is not devastated about losing Peter like the little mermaid in the fairy tale > She thought he was her prince charming who fills her days with joy > but turns out the one who actually brings joy to her was Amy all this time. All and all, the confession by giving the jar of cookies has been well-developed and foreshadowed since chapter one, and the author makes it so poetic it rhymes.
Being a sociopath, Bibi is really sharp and rational, I think people might just forget it at some point.