So, the President is legally allowed to just kill anyone, but if anyone is resisting, even just saying “if you kill my uncle’s nephew’s father’s roommate and I will seek revenge”, it’s
18 U.S. Code § 1751 - “Presidential and Presidential staff assassination, kidnapping, and assault,” which covers attempts to kill or inflict harm on the President.
18 U.S. Code § 871 - “Threats against the President and successors to the Presidency,” which criminalizes threats to kill or harm the President.