Injustice
The theme of injustice is interesting because it is very individual. What some see as injustice is perfectly normal to others. That's why authors have to look for universals. These are situations or ideas that most people see from the same point of view.
For example a rich man walking past a bum in the street is a picture of injustice. A woman being beaten by her husband for no good reason is a clear sign of injsutice
Writers have to make sure that they make their audience see the injustice. They often use certain techniques like point of view to make the reader realize the seriousness of the injustice.
It is interesting that "the law" is often associated with injustice. It should be the tool to enforce justice but in many cases it is manipulated to keep injustice in place. Many movies are based on this theme. We often see heroes who can no longer rely on the law and have to take matters into their own hands. Think of movies like Cape Fear, The Fugitive, Fatal Attraction etc.