In the far past there was not much killing of civilians in war. At least not like the weapons of mass destruction that we have now. The citizens might not even like the country that they are in. They may not support the political agenda of the country. The country could be corrupt and mistreat the people.
There is a distinction between armed combatants and unarmed civilians for a reason: what happens when your nation, your community, your people are the target of an opposing force? Do you want your own kind targeted indiscriminately? Do you want to create a culture of revenge and hatred that will only prolong war?