In no US jurisdiction are you allowed to cut out someone's heart and store it under your floor. You're also not allowed to threaten someone that you'll do this to them, or incite others to do the same. You're not allowed to get on a street corner and say "The time has come to find Bob Jones, drag him out of his house, and cut his heart out of his chest."

You are, however, allowed in all jurisdictions to write a story like Edgar Allan Poe's "The Tell-Tale Heart" where the narrator does the same.

The question that arises in rap music is, when the rapper says, "Imma kill Bob Jones and cut his heart out his chest," is it more like the former or the latter? Is the "I" the rapper, or like Poe's narrator? It's a hard question for the law, because yes, the distinction between a novelist and a rapper is deeply tied into structural discrimination along lines of race and class, but also there are many more rappers than novelists who become the perpetrators and victims of violent deaths.