Why is the murder of Mr. and Mrs. Garstone important to the plot in the story, The Martyr?

The murder of the Garstones is important because of its political implications: "Perhaps this was so because they were the first European settlers to be killed in the increased wave of violence that had spread all over the country. The violence was said to have political motives." The widespread news and "talk" of their murder and betrayal by their own "houseboy" is significant because it creates an atmosphere in which Mrs. Hill becomes afraid of Njoroge, her own "houseboy." It is this suspicion that ultimately leads her to shoot and kill him in what she believes to be an act of selfdefense.