The Munsters S01 E10 Autumn Croakus

Grandpa is feeling lonely and depressed so he contacts a matrimonial agency to find a mate. The agency sends over a nice looking elderly woman who find Grandpa to be delightful. She turns out to be the Black Widow: she makes her living by marrying her suitors, getting them to sign a life insurance policy naming her as beneficiary, and then murdering them.