Question about the ending to Gone Girl (2014) SPOILERS

She has a history of manipulating him and is carrying his child. There's really nothing to figure out. He knows she's capable of killing the child to spite him, and he doesn't care if that would "make her look worse" than him. He's not trying to win the movie, because he doesn't know he's in a movie.

Well I guess I feel maybe I didn't buy his decision cause he made it so quickly. Perhaps if after she got back home, there was an extra half an hour left for him to go through more develop to get this stage, then I could see a gradual change, rather than have him make the decision so quickly, but maybe I just need to watch it again?

But I guess I just got to accept it as making sense, even if it's hard to swallow. It's still a pretty good movie of course .

There is another thing I wanted to ask about in the plot.

If they checked for DNA, sure, but that's expensive. Contra some TV shows, law enforcement can't just run every test on every sample just in case. They triage.

True, but since Ben Affleck's character thought this whole thing was a set up, wouldn't he and his lawyer demand a DNA test though and even pay for one themselves?