The character Meursault in Albert Camus' The Stranger is on trial for murder. There is no question concerning whether he committed the murder. The second issue is whether the murder seemed justifiable as self defense and whether he was in immediate danger in the second encounter with the Arab. Thus your essay will need to focus on whether, despite being guilty, he received a fair trial and was assigned a penalty in proportion to what you consider the severity of the crime.
The first issue you should address is whether his atheism prejudices the court against him. The episodes with the priest, especially, suggest that this may have been a factor in his sentence. The magistrate also seems very strongly prejudiced against Meursault's lack of religion.
Next, the French were the colonial power the ruled Algeria, and were deeply resented, a resentment that lead to the Algerian War of Independence from 1954 to 1962. Colonialism may have led to a certain culture of impunity that affected the trial.
As you write your essay, you should start with an introduction which sets forth what you consider the criteria for a fair trial, and then in your body paragraphs focus on whether this trial meets those criteria.