Threatening to "go kill ni**ers for a high score" is likely illegal.
That shits on /pol/ right now, and is the root of supporting extremism to the point of violence and murder. And last I checked, making death threats is explicitly not protected speech.
However, "go kill ni*ers for a high score" is not only not a true threat, it's the kind of speech that Brandenburg v Ohio, which established the current incitement-to-imminent-lawless-action test, explicitly said was 100% protected free speech.
Pretty much anything less than leading a mob and telling them who is their next lynching target will be ruled protected speech.
Your test sentence fails the Brandenburg test: it is not "directed to inciting or producing imminent lawless action", nor "likely to incite or produce such action."
That shits on /pol/ right now, and is the root of supporting extremism to the point of violence and murder. And last I checked, making death threats is explicitly not protected speech.