No, there's a set of statutes that lay out what murder looks like, and ultimately it is up to a jury of your peers to determine if what you did satisfies the criterion. That's in fact exactly why the jury system was invented, because reasonable people can disagree, so the assumption becomes that "if a reasonable plurality of people DO agree, there's a good chance it is a good enough standard by which to act."

The subject of murder is not an appropriate analogy here, really.

Why is that not perfectly analogous? The law can describe what is and isn’t hate speech, and courts and juries can decide individual cases when necessary. This is the same for all criminal laws. The fact that not all people will agree what is and isn’t a violation of a given law at a given time is simply not a valid argument for why a given law shouldn’t exist.