Certainly, more ruled-as-murders per population unit happen in urban areas, but I'm aware of no statistics by urban/rural residence of killer (where a crime happens doesn't tell you where the criminal is from, and people who live in rural areas obviously can commit crimes in cities.)

And rural areas have higher death rates and it's at least plausible that the decreased population density makes it easier for evidence of homicide to be concealed and murders to be passed off as death by other causes, so the relative rates of ruled-as-murders may not accurately reflect the relative rate of actually-is-murder.