What is the law that says armed civilian is legal to be shot down in combat?

I once read that any armed civilian who pose threat to soldier is legal to be shot down. I read that somewhere in the internet, and I thought I saw this when studying Civics, too.



But I forgot what/which law explains that thing. Can someone help? Thank you.What is the law that says armed civilian is legal to be shot down in combat?
Law? The law of common sense.



I served in Vietnam with the River Patrol and every single person we killed was wearing civilian clothes. I could care less if they are in uniform, naked, wearing a jock strap or a tux. Shoot at me and I will kill you if I can.



My kill count was 43 all in civilian clothes.