The Munsters (1964)

S1E10 Autumn Croakus

Feeling isolated and downhearted, Grandpa decides to reach out to a matrimonial agency in search of companionship. The agency introduces him to a charming elderly woman who finds him quite appealing. However, unbeknownst to Grandpa, she is the infamous Black Widow. Her sinister profession involves marrying her partners, persuading them to take out life insurance policies that list her as the beneficiary, and then orchestrating their deaths. As their relationship unfolds, Grandpa remains blissfully unaware of her deadly intentions.