This Was a Woman (1948)

The woman here is Sylvia Russell, who is a bit power mad and has some set goals. In order to achieve those goals she is sadistically devoted to her son and daughter, able to possess and direct them. She subtly ruins her daughter's marriage by corrupting a servant girl and, then, uses her to compromise her son-in-law; slowly breaks her bumbling, unambitious husband's confidence, finds in her husband's executive friend the man she desires, and then poisons her husband in their quiet English manor.