Can law enforcement/military shoot to kill for merely making a phone call that is deemed treasonous?

Question

Warning: This question contains spoilers to the film Arrival (2016). If that bothers you, move on now.

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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.

Answer 1

Treason, per se, is probably not a valid reason to shoot someone on the spot given how that crime is defined in the U.S. Constitution.

But, keep in mind that in Arrival the situation has been defined as a military operation. As a result, the relevant body of law would be the law pertaining to actions that a military officer may take to carry out a mission which has been stated broadly by the President (as commander in chief) or by Congress in an authorization for the use of military force, or both.

Thus, the authority of the officer in this situation would depend upon the rules of engagement and rules for military discipline for dealing with civilians present on base in a military operation for achieving their lawful mission. Under these circumstances, the question would not be the usual self-defense or defense of others analysis, but whether the order given was a lawful order in light of the mission.

Naturally, we don't have the exact details of the legal authority that was given or the definition of the mission in the movie, but "all necessary force" to complete the mission would not be an unusual set of rules for a high priority, existential national survival military mission, and in that case, the lawfulness of the order would depend upon whether the military officer giving the order reasonably believed that shooting someone was necessary to accomplish his mission. If so, he would be authorized to give the order and it would be lawful.

Answer 2

The answer to the titular question is, "Yes, and it need not even be a treasonous phone call."

In the United States anyone can be justified in using lethal force if they can establish that, in the moment, it was reasonably necessary to prevent imminent, grievous, and unlawful bodily harm to themselves or to another person. (The italicized terms may vary by jurisdiction, but AFAIK the same idea and meaning are codified everywhere.)

How can making a phone call cause imminent death to another? Examples that come to mind: One could be keying or relaying the code or command to detonate a bomb. One could be confirming the name of an undercover agent who will be summarily executed if outed.

In the particular case described in the question, one would expect the soldiers to shoot if commanded. When military enlistees are handed a gun and given an order to shoot, they are expected to not put a great deal of thought into whether to follow that order. Unless it is a clearly unlawful order, that is, one that they know violates military law, standing orders or rules of engagement, they will be disciplined for not shooting on command. (It is the commander who might face a more detailed trial at some later date into whether the order was lawful given the circumstances.)