Yes, there were few standing armies in the ancient era, but that has little relevance to the issue. Do you think that attacking soldiers could not make a distinction between say a small child and a grown man? The slaughter of non-combatants was par of course and had nothing to do with blurred distinctions. It has often been the policy to slaughter, rape, terrorise and enslave your enemies populace. If anything, Western Europe got off lightly during WWII compared to the experiences of their ancestors.
Also, I did not exclusively reference the ancient period. As I said, even as recently as the early 19th century civilians would be targeted. In Spain whole towns were put to the sword by the French in order to stem the guerillas. In fact, look a the War in the Vendée.