Wrong. So wrong. Ever heard of due process? No state has the power to declare legal the deprivation of life, freedom, or property without due process of law. That the states can formulate their own scheme for prosecution and punishment does not liberate them from the federal guarantee of the underlying natural right upon which the definition of murder depends. So until they start finding babies guilty of capital crimes, you will not soon find any state declaring murder legal. How bogus we can get when trying to justify a popular but failing legal strategy!
Due process is what is due to the accused in a criminal trial. If, e.g., murder was legal then there would be no accused, no trial, and no occasion for due process. Constitutional protections protect us from the government, not each other.