A Mother's Love
The main character in the movie is Mildred, and she is both a strong woman and a fierce mother. Her love for her daughter has turned to the most extreme grief, coupled with a feeling that nobody is trying to find justice for her, or punish the person who killed her. She sees this as a slight in that it implies that Angela is not important enough for the police to go the extra mile for. Everything that Mildred does is for Angela. She subjects herself to quite awful harassment and bullying because of the billboards but she knows that she is right in putting them up and is resolute in her determination not to be bullied into silence. The film also shows that there is no stronger love than that of a mother for her child.
Revenge
Is Mildred looking for justice of revenge? It's sometimes hard to tell, but there also does not have to be a difference between the two because in the case of Angela's murder, justice and revenge would look like the same thing to her family. Mildred wants the perpetrator of the crime caught and brought to justice, but when it appears that this is not going to happen she realizes that it is going to be down to her to both catch and bring him to justice.
She is also furious with the police department for failing to apprehend the murderer and for failing to put in much effort at all in identifying him. This anger turns to revenge; she puts up two of the billboards partly out of revenge and party in an attempt to light a rocket under the police department; unfortunately when the metaphorical rocket fails to work she turns to violence instead in her need for revenge.
Loyalty
There is a theme of loyalty in the movie; Mildred, of course, is loyal to her daughter, and to her memory. She is also loyal to what she feels is her daughter's worth as a human being, and this is why she is angered by what she sees as the disrespect of those who have not caught her murderer yet.
The townspeople are loyal to Chief Willoughby' his illness is an open secret and the prevailing opinion is that Mildred is kicking a man when he is at his most vulnerable. Officer Dixon is also incredibly loyal to the chief; while he is alive, he is loyal to him in a practical sense, but after he has passed away he is loyal to his memory and also to the man that he was. Dixon changes himself after realizing that it is disloyal to represent Willoughby in the way that he has been.
Sex Crimes Against Women
The film suggests that sex crimes against women are not taken as seriously as they should be; one of the main ways that this is shown in the film is through the townsfolk's universal support of the chief of police rather than support for a grieving woman whose daughter has been raped and murdered. The fact that the citizens of Ebbing have more sympathy for a man who has not succeeded in doing his job and identifying a suspect than they do for a woman whose daughter has been brutally slain suggests that they are less offended by the slaughter of a young woman than they are by the slander of a police chief. There does not seem to be a sense of public outrage over what happened to Angela, or a sense of identifying with her mother. There also seems to be no public pressure on the police to find out who committed the murder. This all hints at a society in which crimes against women are accepted as a normal fact of life rather than made a priority in solving.
