“Unconstitutional laws are null and void,”
Isn’t murder up to the states to define?
Yes and no. Since the 14th amendment, it is considered a "civil rights violation" and so therefore the Federal government believes it has jurisdiction, at least if it comes to a group that the Fedgov regards as "persecuted."
I guarantee, if such a law was passed, the Fedgov would step in and prohibit it as a "civil rights" violation.