Violence Motif
Although this is not the traditional shoot-'em-up kind of violent movie, there is nonetheless an underlying motif of violence that is both its foundation and its undertone throughout. The movie is centered around an act of extreme and psychopathic violence, the murder, and the subsequent rape, of a young woman. A seemingly innocuous act, that of putting up a billboard reminding people of the crime and the need to solve it then spawns the most random acts of violence directed at the victim's mother; Mildred is threatened multiple times by complete strangers, and is also threatened by her dentist; she defends herself against him with further violence and drills a hole in his hand with a dental implement.
Officer Dixon is also a violent character; most of his violence is driven by his alcoholism and the fact that he is an angry, bitter drunk. He assaults the man who rents the billboards to Mildred, and is also violent towards anyone who supports her.
The movie ends with the suggestion that more violence is to come as Dixon and Mildred, now working together, set of for Idaho in search of a man they know to have committed a rape and murder with the intent of killing him.
Family Loyalty Motif
Two of the three main characters in the film show great loyalty to their families, and this is a motif that persists throughout. The plot is driven by Mildred's loyalty to her daughter; the chief of police, of whom she is extremely scathing, is also a man who is tremendously loyal to his family. On what he has decided to be his last day alive he spends with them, and gives them a wonderful day filled with love and happy memories; he kills himself at the end of that day because he does not want them to have to watch him die of cancer. He is loyal to them in that he does not want them to experience any suffering, and he puts their needs above his own.
Billboard Story Allegory
The story itself is an allegory of a real life murdered girl, and the billboards put up in order to catch her killer and to berate the police for their failure to make an arrest. Martin McDonagh assumed the person who put up the billboards to have been the victim's mother although he had neither proof nor confirmation of this. He allegorized the real-life story in the film, including stating definitively that the person who put up the billboards was a grieving mom.
Billboards Symbol
The billboards are a symbol of Mildred's grief about her daughter's murder. She misses her daughter and wants the world to value her life as much as she did. She feels that this is not happening and that nobody is treating the case with any urgency, turning her grief into frustration. The billboards are a symbol of this and the way in which she feels like she has to take matters into her own hands in order to solve the case and afford her daughter the respect that she deserves.
Billboard Symbol
One of the billboards tells that Angela was raped as she was dying. This is a symbol of the extreme psychopathy of the individual who committed the crime; she was not murdered because she accidentally caught sight of her rapist, or because the rapist was afraid she could later identify him; she was raped by a man who committed a murder and had necrophiliac tendencies in that he wanted to rape a girl during her dying moments.
