True, but modern thinking doesn't necessarily preclude methods of war from being warcrimes. The problem comes in that sometimes... what soldiers do to civilians can be as nothing compared to what the civilians do in return.
I point you toward the Peninsular War, where Spanish peasantry were just as guilty of torture and what one would conventially call 'warcrimes' as French soldiers. Much as France and Britain hated each other, French soldiers wanted to be captured by British soldiers more than anyone else given the circumstances.
Anyway, I digress. This is one of those few occasions that one can talk about Clausewitz's trinity, the first one being the relevant one (though there would be some interplay with the second): violent emotion; and the involvement of chance and probability. Some alien creature comes down and kills your family in front of your eyes with the dispassion of a serial killer, you're either going to shit yourself and go nuts or go nuts and think vengeance for a good while. Either way, there will be the inevitability of the aforementioned violent emotion, whether you, another witness or someone you know to whom you recount the scene, it will be felt. While on the individual basis this is understandable, even justifiable, stemming it on the broader scale is what we need to consider.
Now, the idealised purpose of war (IMO - and please note the use of the word 'idealised') is to create a better peace than the peace that led into the war, regardless of motivations of individuals. This can be achieved in only two ways: annihilation of one or both combatants; or removal of the desire for war and any broad possibility for future enmity. The latter is the holy grail of peace treaties and the difficulty in its execution is such that it has only been done on a handful of occasions. The Treaty of Versailles (1919) is a great example of how not to do it, the American occupation of Japan after the War in the Pacific is a great example of how to do it correctly (or at least from what little I know of it).
In the example we thus consider, we have to take into account the fact that the alien mentality is simply and purely annihilation, and considering that is the approach of the soldiery, that is necessarily the approach of the citizenry, otherwise one would wonder at their political system. So how does an occupying soldier deal with civilians when the civilians will take any and every opportunity to kill that soldier, regardless of their own safety?