Warning: This question contains spoilers to the film Arrival (2016). If that bothers you, move on now.
In the 2016 film Arrival, humanity confronts a crisis with the arrival of a dozen alien spaceships. The US military brings linguist Louise Banks out to the one in Wisconsin, where she leads a team at a nearby temporary military installation and figures out how to communicate with the aliens.
Near the end of the film, higher-ups in the military decide to close and evacuate the installation. As they are preparing to close the communication links, a soldier notices that his superior's phone is being used for a satellite call to China, presumably a call to the site of another alien ship there. After a brief search, Louise is found to be the one making the call, and confronted in a small space that used to serve as the decontamination chamber, along with lead scientist Ian Donnelly who tries to get her a few more seconds for the call. The supervisor and two armed guards are pointing guns at the pair. The supervisor asserts that the call is treason, and that if she doesn't drop it now they will shoot. Just before shots are actually fired, she finishes the call and surrenders. Neither of the two had firearms nor did either pose an immediate physical threat to the others on the base, though the superior was clearly concerned about what the full consequences of that call could be.
Had either or both of them been shot dead, would the killing have been ruled justified after the fact? If not, what if any consequences would likely apply to the shooter(s)?
Note: I am not sure if military or civilian law would apply, but even if the correct answer is military law, it'd be nice to know the equivalent answer if these characters were facing civil authorities (e.g., local law enforcement), as could be the consequence for e.g., unauthorized international cooperation in the face of an epidemic or similar crisis.
Answers to any other factual questions/assumptions about the situation would be taken from the film.