...except SC doesn’t have degrees of murder.
Quite right. Just went and looked up the SC statutes:
SECTION 16-3-10. "Murder" defined: "Murder" is the killing of any person with malice aforethought, either express or implied.
SECTION 16-3-50. Manslaughter: A person convicted of manslaughter, or the unlawful killing of another without malice, express or implied, must be imprisoned not more than thirty years or less than two years.
So unless people believe that Slager sought to kill Scott, I can't see why they would charge him with murder. Manslaughter is the appropriate charge.