The Ed Clum murder case
White and Clum were from the same small town in New York and were members of the same unit of the Union army during the Civil War. Returning home after the war, they remained friends and took up residence close to each other. The older White had married during the war, and Clum entered into wedlock with a carefree girl named Lottie not long after his discharge. White was apparently a frequent visitor in the Clum home, and he and Lottie soon started carrying on together, right under Clum's nose. White's wife eventually killed herself because of her husband's shenanigans, but it didn't stop his philandering.
When Lottie's health began to fail, she went to live with a sister at Lebanon, Missouri, but White soon followed and took her to a farm he had purchased south of Pierce City in Barry County. Clum showed up and was introduced around Pierce City as Lottie's brother, presumably to protect her reputation. He took her back to Lebanon and continued to New York with the understanding that she would join him as soon as her health allowed her to make the trip, but instead she went back to White's Barry County farm.
After Lottie died during the winter of 1885-1886, Clum made the trip back to Missouri again and resumed the charade he'd started the previous summer, passing himself off as the brother of White's deceased "wife." Folks in the area considered him and White not only brothers-in-law but also good friends.
Soon White started romancing a 17-year-old neighbor girl named Ella Bowe, and when the couple announced plans to get married, it was more than Clum could take. He killed them both with two shotgun blasts apiece. Clum was convicted of murder and hanged on the square in Cassville in the spring of 1887.
The full story of the Clum murders constitutes a chapter in my "notorious incidents" book.