It is, and many wrongly agree that is the correct position to take. Up to the States to decide.
However the 14th Amendment is still applicable:
No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
By that interpretation murder is a federal crime and not a state crime. Do you really want that?
The goal for conservatives should be to push more “governing” down to the state level. The recent abortion ruling does that.
Do you really want to continue the “statist” march toward all governing done at the federal level? States devolve to just historic boundary lines on a map.