The French resistance only put on arm bands during the liberation of Paris to avoid being shot by the allies.
Your insignia argument doesn't wash.
And the Nazi's were convicted for executing non uniformed resistance fighters, including spys requited by British intelligence.
The Nazis were convicted in Nuremburg for crimes which were no crimes at the time they were committed (legally). There was no law allowing or forbidding anything on or out of the battlefield. But the idea of Nuremburg was, that some things simply cannot be right (such as executing civilians without a trial). Shortly after that, the Geneva Conventions were written.