You're thinking about it wrong.
Murder rap is an offshoot of battle rap. The idea being how can I envoke disgust and humiliate the competition at the same time.
If they go straight to rape and genital mutilation then they're just sub-par murder rappers. Good murder rap makes you go "well got-DAMN "
Like this:
Never heard his music until this but the entire time I thought it was pronounced like rhino, not Reno.