So if a State decided to legalize murder that would be OK under the 10th amendment?
Yes, under the Constitution, the Federal government has very limited, enumerated powers.
The States are free to make bad laws or good laws or no laws.
Wife-beating, abortion, rape, clubbing baby seals, killing puppies, etc., might be evil things to do, but they are not Federal issues.
If you think the States are too unjust to handle these issues, then why stop at the Federal level? Shouldn't the U.N. make these decisions, because otherwise what if the U.S. legalized murder?
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.