Federal courts do not have jurisdiction over murders by default. There are two obvious cases where they do (murder of a federal official, murder on federal land) and a shaky third category of "murder pursuant to another federal crime". In the past that third category has been used to charge organized criminals, anything that touches the interstate commerce act (drug trafficking/contract killing/etc), and terrorism.

Charging Mangione with federal murder connected to a federal stalking charge was relatively unprecedented, but they might get it to stick on appeal. Stalking is threatening but maybe not inherently violent, but that seems similar to bank robbery (where FDIC insurance is frequently used to grant federal jurisdiction over involved homicides).