Except on federal property, Congress is NOT authorized to make murder a federal crime. If you believe otherwise, please provide the specific article and section which grants fedgov that authority. Read the provisions of the Tenth amendment before you go looking.
The ability of the Federal government to prosecute for murder - in pursuance of the Federal government's authority to make all laws necessary and proper to its governance - does not in any way hinder the power of the individual states to try any individual for murder.
The individual states retain the right to prosecute any perpetrator of any crime within their borders.