4 ideas
Tobias Greene's will requires his quarrelsome heirs to live together in the decaying family mansion for 25 years to inherit, and they are shot or poisoned one by one. The killer is Ada, the adopted daughter everyone pities, who copies her methods from real cases in her late father's locked library of criminology books, including Hans Gross's work.
In the first attack, one sister is killed and Ada is only wounded. Staging your own injury is a strong alibi because sympathy stops suspicion, and the pattern of 'the killer is picking off the family' puts the survivor among the next targets rather than among the suspects.
Instead he treats each murder as a creative act that shows its author's temperament, imagination and limits. He asks which person in the house is psychologically capable of this particular style of crime, which narrows the suspects when the material clues have been deliberately planted.
A legal condition that forces people to share one household as the price of their inheritance removes the normal release of simply leaving. Grievances and mutual suspicion build up with nowhere to go. The house becomes a closed system in which everyone has motive and opportunity, so the conditions that produce violence also provide its cover.