In both examples, the President used his obligation to take care that the laws be faithfully executed, after it became clear that "the ordinary course of judicial proceedings or the powers invested in the US Marshals" were inadequate to deal with the situation.
Murder is not a Federal crime, even if it's done with an AR-15.
Murder is a Florida offense, the courts are open in Florida, and unless I am mistaken the ordinary course of judicial proceedings still sends murderers in Florida to the electric chair, so I stand on my view that this is not an FBI matter.
Murder most certainly can be a federal crime under certain circumstances.
https://www.wklaw.com/10-ways-murder-becomes-a-federal-crime/
I myself during my working years would have been covered by
18 USC section 1114 had I been killed while performing my duties.
FBI has long history of assisting state and local govts with crime issues.