with this:

from JMA
If he is armed... you shoot him.
Lots of folks run around armed in many areas of the world; e.g., Pashtunistan, Dayuhan's Mountain Province and even Michigan. So, some indicia of hostility must be present.

Now, if where you are people are not armed (unless they are good guys or bad guys), being armed is itself an indicia of hostility. Also, if the person is PID'd as a member of a designated hostile force, whether he is armed or unarmed, hostile or not hostile himself, are not material to the shoot.

As to dealing with war criminals in the field, I'd much prefer on-site military tribunals over not taking prisoners. Once upon a time (through WWI per our Articles), the field commander could convene a board (usually 3 or 5 members) and try war criminals on the spot.

Regards

Mike