However, I still struggle to overcome the vapid nature of the film's intent. What is the arc of these characters? Where does the story go after the film's conclusion? What do the characters learn? What do we learn? This is easy for me. We're so used to a film with a premise like this being about vengeance, but it's actually about healing, about moving on and making peace with the fact that the dead are dead and any unresolved issues are likely to remain unresolved.
Two things that I keep coming around on.
1. How come we never learn what happened to the daughter's remains? Was she buried or cremated? Is there a plot or a final resting place? I ask that because the billboards become her memorial, and we see that with Mildred bringing fresh flowers to the billboards and her conversation with the deer.
2. Isn't it interesting how close the Billboards are to Mildred's house? And how we don't get that information until the film is almost over, when Dixon shows up with the results of the DNA test? Why do you think McDonagh did that when there could've been shots of Mildred watching the signs go up from her front porch. We could see her watching Dixon learn about them.
I don't think either of these decisions are bad, they just show McDonagh making a different type of film.
I think it goes deeper than McDonagh making a different kind of film. I think the film was really about Mildred and Nixon being validated. Mildred wants to be supported as a mother who lost her daughter to murder and wants the law enforcement to continue investigating. Nixon wants to be supported as a police officer, who in spite of his poor judgement and unruly behavior, has the capacity to be a good person. By act three Nixon reads a letter from Willoughby that confirms the idea that he is a capable person who needs to grow beyond his fatal error. And Mildred, well, she gets support from Nixon (even though he's no longer a police officer at this point) in the investigation even though he comes up empty handed.
Even though the last scene appears to cut away before something critical will happen, I think the film ended perfectly. Both Mildred and Nixon have forgiven one another. Nixon has forgiven Mildred for his injuries, and Mildred has forgiven Nixon for being an investigative dingbat. As they contemplate whether or not they are going to commit murder, we the audience see for the first time a sense of peace on their faces. Essentially, the idea they are going to kill this guy because he raped and killed a girl at some point (probably while overseas) is a red herring.