Well, these are ballads, death is a common subject... Francis Child's collection of traditional English and Scottish ballads and their US variants , now known as the Child Ballads ,has lots of dark ones. Murder of girlfriends, murder of an engaged sister by a jealous un-engaged sister, murder of a commoner seduced by a noble woman and caught in her bed...it's all in there. These themes were also in broadside ballads, written to be sold in fairs in the 17th and 18th c. , set to popular tunes. Some of these commercially-written songs that entered the traditional repertoire were written about an actual crime. Folklorist Cecil Sharp , one of the first of an army of "ballad-hunters" to work in the US, noticed that many of these traditional ballads were still commonly found in the South and the Southern Appalachians, and that region ( thanks to Monroe, Scruggs and Flatt) is where Bluegrass came from. Flatt & Scruggs, the Foggy Mountain Boys, etc. simply adapted the songs they'd grown up with to their own style.

The killing of girlfriends can seem rather mysterious, because it seldom is assigned a motive: one moment, the happy couple is walking along the ocean, the next moment the boyfriend pulls a knife and stabs his girl. The best explanation I've heard for this is from folklorist Michael Kline: there was likely an unwanted pregnancy, a rape, or a refusal of sex, any of which was impossible to explicitly state in mixed company or in front of children. Bloody murder, of course, was perfectly fine to describe.