I suspect that there are trends to follow when looking for murder ballads. There are lots of real and imagined stories in older songs, and there are a few modern reports of murder like Bob Dylan's "The Lonesome Death of Hattie Carroll" (more info here). I don't listen to rapper music (unless it is vibrating through my pickup at a traffic light, coming via a nearby vehicle with a cranked up sound system), but there is a lot of violence in it and I would hazard a guess that you will find occasional accounts of murder there.

SRS