We needed a constitutional amendment to end the abominable practice of slavery.
Ironically, your position, if carried over from the matter of life to liberty, ie slavery, would posit that even with an amended Constitution outlawing slavery that the states have some supposed legitimate power to allow it anyway.
Because, our existing Constitution, unamended, explicitly forbids the killing of any innocent person, in two amendments...and imperatively requires every state to provide for the equal protection of the laws for every person within their jurisdictions.
pos·ter·i·ty noun 1. succeeding or future generations collectively"We the People of the United States, in Order to...secure the Blessings of Liberty to...our Posterity, do ordain and establish this Constitution for the United States of America."
[po-ster-i-tee]