Poorly executed adaptation and ending discussion.
I haven't read the book, but I felt it ALOT that this was a bad adaptation. It's choppy and clumsy and some parts are very heavy handed and other things are just glossed over. Overall I thought it was very "unbalanced" if you can say that... It takes a great director to do it right I guess, and Gillian Flynn needed Fincher to do her justice. Gone Girl was a perfect movie.
How did Libby find out that Polly killed her sister btw? I get that she realized that she was there that night and wasn't being honest about that, but nothing else pointed to her having killed someone.