I'd like to preface this post by saying that I absolutely loved the movie and am in that camp that just wishes the film had a stronger third act.
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I think the core theme of the film would have resonated better if Ares ultimately wasn't present and remained a
MacGuffin.
I loved seeing the horror dawn on Diana's face when she realized that Ares wasn't controlling Ludendorff's soldiers and they were operating under their own volition. Coupled with Steve's speech to her on the watchtower, it was a powerful moment.
Her whole drive in the film is to kill Ares. She left her home, traveled hundreds of miles, killed dozens of men, witnessed the death of an entire village to chemical warfare and at the end of the day the war was mankind's fault - not a deity. It's a hard lesson for someone as naive as her to embrace.
I feel like the finale of the film should have been about her putting her disillusionment aside and resolving to save these people from their own folly and prevent a catastrophic attack from spiraling the war into a second-wind.