I Could Murder Her — E.C.R. Lorac

Muriel Farrington is a domineering woman who, unfortunately for them, has her entire family living with her in her stately home. She tries, often successfully, to run the lives of her children, her stepchildren, her in-laws, and her husband, and she seems to be despised by all except her husband and one son.
When she is found dead one morning in her bed, the family doctor, who is old, ill, and hasn't been very able for years, is unable to attend and bestow a certificate, which he would have done without investigation or thought.
A younger, more able and perceptive doctor has to be called in, to the shock of whoever the murderer was, and he does not find the death natural.
A hypodermic puncture in her arm leads him to believe, correctly as it turns out, that someone has injected insulin into the woman. Since she was not suffering from diabetes, death was the inevitable result.