Kind Hearts and Coronets (1949)

In Edwardian London, Louis Mazzini feels the need to write his memoirs due to his current circumstances. He grew up in a humble, working class household despite his mother being from the wealthy D'Ascoyne family, who disowned her when she, in their eyes, married beneath them in Louis' father, who Louis doesn't even remember in he having passed when he was an infant, being a poor opera singer. She was in direct line to become the Duchess of Chalfont. The D'Ascoynes refused even to recognize Louis' existence as a result. The last straw in this matter for Louis was when they refused her last request to be buried on the Chalfont estate upon her passing. But it wasn't until vain and opportunistic Sibella Hargrove, with who he was in love but was instead going marry his rival Lionel Holland who could afford to keep her in the lifestyle to which she aspired, that Louis decided to pursue what he considered his rightful place in the D'Ascoyne family and the title of Duke of Chalfont. Pending other issues such as children being born into the family by other heirs, Louis decided to achieve his goal by not only needing to eliminate the Duke but the then eight other D'Ascoynes who were in line as Duke or Duchess before him, the easiest way in his mind to commit the murders being either to ingratiate himself into their lives or learn in great detail about them to do the acts easily but without detection. What may have thrown a wrench into the proceedings is falling in love with Edith, a D'Ascoyne by marriage and who is/was in opposition to Sibella in his life.