The 14th amendment forbids any state from enforcing any law which abridges the privileges or immunities of citizens of the United States. But then “state’s rights” confrederates aren’t big fans of the 14th amendment are they!?
There is nothing in the Constitution which requires states to make murder a crime. If the state wanted to it could legalize murder, so long as everyone had the same right to murder. Of course it is extremely unlikely to happen, and it would be unjust, as was Florida’s treatment of Terri Schiavo.