[BenG]

I'd like to nominate The Deadly Dowager by Edwin Greenwood.

Spoiler:

According to a contemporary review, it was “quite the jolliest crime story that has come our way in many moons.” Yet it’s not at all comic in the manner of, say, a Donald Westlake caper about Dortmunder and his lovable gang of burglars. Black humor, gallows humor, sardonic humor — these better describe the overall tone of Greenwood’s tale of the 83-year-old Dowager Duchess Arabella, Lady Engleton, who decides to do away with a handful of her inconvenient relatives.

Arabella becomes a serial killer for what she believes is the best of reasons. Her own two sons having died, one in the Boer War and the other in the Great War, and the de Birkett family’s fortunes having precipitously declined, she has taken it upon herself to establish her 20-year-old grandson Henry in a manner befitting his noble station. Initially, she persuades various childless in-laws — a dotty clergyman, a blustery India hand, a Harley Street doctor, a stupid businessman, and a pair of sisters, one repulsively fat, the other mousy — to allow her to insure their lives, making Henry the beneficiary. Arabella will naturally pay all the fees against the day — no doubt quite distant, of course — when each finally shuffles off this mortal coil. While maintaining a demeanor of sweetness and innocence, she then starts killing them off, one after the other.
https://www.washingtonpost.com/enter...1a1_story.html

https://www.goodreads.com/book/show/...om_search=true