In 1930s mysteries, all sorts of motives were credible which aren't credible today, especially motives of preventing guilty sexual secrets from coming out. Nowadays, people sell their guilty sexual secrets.
“Seek not the favor of the multitude; it is seldom got by honest and lawful means. But seek the testimony of few; and number not voices, but weigh them.”
“It is not necessary to have enemies if you go out of your way to make friends hate you.”