Interestingly enough this started under Garland...where the government alleged the premeditated planning and stalking of his victim with the intent to murdering him, which involved crossing many state lines (what they called "interstate stalking" and "use of interstate facilities" causing death) , and for using a silencer on a gun used in the commission of a crime are federal crimes.
As is true in most crimes, the federal violations are more severe. And in this case NY ended capital punishment...Trump reinstated it at the federal level...something, ironically, I guess Garland didn't expect when he filed federal charges on Mangione.