I suppose Rudy Vallie's version of "With Her Head Tucked Underneath Her Arm", Phil Ochs' "The Ballad of Alferd Packer" or Bert Williams' "Somebody Else Not Me" might be too humorous to be considered a valid murder ballads for your research. What about contemporary ballad writers of songs about murder? Would these be of interest to you, or are you limiting your research to only those songs that have been written long ago?
A little off topic but as a writer I've pondered these questions. Is there a reasonable time frame that limits a writer's exploration of the facts, or lack there of, when trying to create a song about an event that is considered heinous, like murder? Do the facts of such an act really matter in the creative process? It seems to me that in this day and age of disinformation, and top secrets we in this country (the U.S.) aren't even allowed to know the facts of so many horrific acts, due to government secrecy. We merely banter back and forth the what-ifs until we're all sick to death of hearing about the subject. We murder any real interest in the truth via our incessant speculation.
That said, I've written a few that might qualify.