spys or saboteurs are unlawful combatants, who do not wear uniform or military insignia and they may be captured after being shot, they do not have the rights of POWs as lawful enemy combatants have. that is international law. didnt you learn that during your time in the army? we had to know exactly whom we were allowed to shoot in which bodyparts how often under what conditions.
as the occupying army is responsible for security he can be a spy in his own country and shot as such. if he was wearing a uniform, he would have more protection; as during WWII the polish liberation army, who was still fighting, allthough they had lost their country to germany and the ussr, wore uniform and was recognized by the germans as regular army.
international law is fun, until WWI it even provided for retaliation against civilians. for every soldier killed by spies/saboteurs/terrorists 10 civilians could be shot, but only 10, if you shot 11, it is a war crime. the german army used to make use of that provision and shot civillians after terrorattacks on the wehrmacht in WWII.