The president cannot lawfully order the killing of anyone, except according to the Constitution and federal law. Under the Constitution, he can only order killing using the military when the U.S. has been attacked, or when an attack is so imminent and certain that delay would cost innocent American lives, or in pursuit of a congressional declaration of war. Under federal law, he can only order killing using civilians when a person has been sentenced lawfully to death by a federal court and the jury verdict and the death sentence have been upheld on appeal. If he uses the military to kill, federal law requires public reports of its use to Congress and congressional approval after 180 days.
Judge Andrew Napolitano