B. Revenge. The motive behind the murder of the author's sister and her husband was revenge for a previous wrong done to the murderer by one of the victims. This would be the most logical and straightforward motive for the crime, and would make the most sense in the context of the story. It is also consistent with the character of the murderer as presented throughout the text. A and D are not likely motives for the crime, as they are not directly connected to the victims. C is not a likely motive because the murderer was not motivated by a desire for knowledge or new experience.