Is the norm in war to only harm enemy soldiers for moral or contractual reasons?

It is clear that the norm in international law to only harm enemy soldiers is beneficial, otherwise there would be many more deaths on all sides. But is this simply a beneficial agreement, like they used to do in a duel, or is it a moral issue? After all, there is essentially no difference between the army and the country that sent them. In a regular war, you fight against a political entity, not against soldiers who came from nowhere.
And what about Gaza? On the one hand, it's civilians, on the other hand, they supported Hamas until they started to suffer. 
And also, do you think that civilian Hamas government officials, who are responsible for infrastructure, medicine, and the like, are also a legitimate target, or only military ranks? 
thanks