This is purely speculation, of course, but I keep wondering if, perhaps, it was someone in the law enforcement community who found PM too inconvenient, worried about what might come to light during the "discovery" phase of the trials. My thoughts keep returning to the shotgun and AR-type rifle used in the murders. These are common enough anywhere, but nowadays, nearly every police cruiser comes equipped with both, as standard equipment.
In my county of residence here, back in the 1970s and 80s, the county sheriff ran the local cocaine trafficking network. Lots of people knew about it, but no information about it ever came out in court. He retired prosperously. What sort of conspiracy might lurk beneath the surface in the Low Country?