Watched it for the first time, have to say that this movie was excellent. So many twists and turns, and the feel,tone, and atmosphere of the movie was Fincher has his best.
I will agree with the poster that said Nick wanted to leave her but you could tell after he spent more weeks with her, that he still loved her in a twisted way... which is why he just flat out didn't go to the FBI and confess what she told him so they could investigate.
I love the ups and downs of this movie.
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From the first act setting up Affleck to be a classic sociopath who committed the murder but acts like he has no idea about it. It ALMOST convinced me. Then the reveal that Amy was still alive. You then get a different feel of the movie that even though Nick didn't kill her, he deserved some punishment for his infidelity. For a while Amy ends up looking like a smart woman who escaped her domestic violence and the a-hole cheating husband gets some karma.
Then you see how much of a psychopath she actually is just from her encounter with Desi.
I loved the pokes at the state of the American media and how once the fingers start getting pointed you're guilty. I also love how Tyler Perry's character came out. Smart lawyer, who knows how to do the damage control, but if he wasn't helping out the protagonist, you'd think he was a giant a-hole.
And probably the most underrated aspect of this movie was the brother-sister dynamic. Man did Margo really become a great character throughout this movie.
I agree with absolutely everything you said. And also ...
[Show spoiler]Yeah, the sister is such a HUGE character. In some ways, she's really a Greek Chorus, representing everything that we as the audience are thinking. Loved her.