I am trying to understand the actions within their time using what Martens wrote.
So, since Martens wrote these things, in this sense, yes, troops in World War Two used similar excuses (to shoot some members of my family, I might add).
To take your OT example, what is wrong with doing the same as someone who would try to understand the Nazi doctrine and view? Or Pot Pol and his followers or those of people in Rwanda or Dafur or whereever - yet those are modern and I am "in the time" to evaluate them using the various international agreements. There is a difference between explaining and justifying.
Yet, it has been said, we should not use the views of today. Instead, one needs to view matters from the time. Others wrote that actions of the time should be measured against Martens (to ensure it is of the time, I used Martens of 1802 and not 1829).
I read Martens; the formulations of Martens are shocking to my modern mind (. It would seem you would be shocked as well (although the formulation of your post suggests you have yet to read the passages of Martens).
If reading documents of the time and trying to understand them makes me "off the wall", so be it. - R
Martens writes on page 286:
SECT. l. Of the Laws of War.
The law of nations permits the use of all the means, necessary to obtain the satisfaction sought by a lawful war. Circumstances alone, then, must determine on the means proper to be employed; and, therefore, war gives a nation an unlimited right of exercising violence, against its enemy. But, the civilized powers of Europe, animated by a desire of diminishing the horrors of war, now acknowledge certain violences which are as destructive to both parties as contrary to sound policy, as unlawful, though not entirely forbidden by the rigour of the law of nations. Hence those customs which are at present called the laws of war.