Kind Hearts and Coronets (1949)

In the hours before his execution for murder, Louis, the 10th Duke of Chalfont, writes his memoirs. In them he details how, though descended from nobility, he grew up poor and had to pull himself up by his bootstraps, career-wise. Then, discovering that only eight members of the D'Ascoyne family stood between him and the dukedom, he systematically started killing them, making the murders look like accidents.