As far as I know, murder by itself is not a federal crime. Murder against federal agents or on federal land certainly is. If the federal government wants to go after a murderer they might go after that person for civil rights violations or something along those lines.
As far as a I know, murder is a state issue. If some state tried to legalize murder, the politicians who did such a thing would be voted out of office and replaced with new representatives who would simply reverse said law.
Once Roe is overturned and abortion law is sent back to the states, each one would be able to restrict or outlaw the killing of unborn babies.
Sad thing is if Roe was overturned I can’t see many States that would outlaw abortion. I sometimes wonder if we’re not all going to hell for allowing the murder of innocent human life.