That closing page really goes there?
Showing the girl was holding the knife the whole time isn't enough of a twist for the page to go in the end.
Reading it's obvious that chronologically the page happens before. As the yandere younger girl was holding the knife the whole time with her left time until she got happy again. The she threw it away or something and pulled the protagonist inside using her now empty left hand.
Really, the real shock twist should be that MC was also a yandere