A high school friend's father told me a similar story from WWII
Towards the end of the war, Hitler Youth as young as 12 would take rifles and snipe at American troops. The Americans would return fire and kill them. He said that he hated killing children but those kids had guns and would've killed him just as dead as any 20 year old German would've done.
The sad truth of the matter is that if you pick up a weapon in a firefight, you become a combatant. It's a bitter truth, but the truth just the same.