Why are there so many photos of atrocities and war crimes being committed? Why would armies document the war crimes they are committing? I imagine cameras back in the day being too large to take inconspicuous photos so I assume the guilty are documenting the acts themselves.
I'm talking about rows of dead prisoners in their mass graves, the concentration camps, ghettos, prisoners being blindfolded and shot etc....
I can speak somewhat to the photographic part of it, as a photographer myself. There were plenty of smaller cameras that were in popular use prior to and during WWII. Leica is probably the most well known of the small sized 35mm camera and they were produced starting in the late 20s and popularized in the early 30s. Here's an example of an average Leica camera:
https://s-media-cache-ak0.pinimg.com/736x/67/0e/c6/670ec60c1ab76d8e4286041990c79022.jpg
Some of the most famous photos of WWII, including Robert Capa's D-Day Invasion photos were shot using Leicas that are smaller than the average digital point-n-shoot today. Canon, Kodak, and Yashica also made cameras of this size that were commonly used. So being surreptitious wasn't necessarily a barrier.
But aside from that, many of the photos of atrocities were, in fact, taken by the people who committed them because those people didn't consider them atrocities at the time. They considered them valid acts of war, ones which they would be rewarded for committing. There is a famous WWII photo of an American POW being beheaded by a Japanese soldier, in which the Japanese officer told one of his men to photograph the event. The same thing happened with German soldiers who photographed the executions of Jews and other prisoners. Many of the scientists and doctors who photographed their experiments also considered the photography to be valid and necessary documentation. You have to remember that at the time these events were happening, the people doing these things didn't consider themselves "guilty" or believe that they were committing crimes, much less war crimes.
And, from the other side, a great many of the war and propaganda photographers, including Robert Capa who I mentioned above, have said that they considered it a moral obligation to show the atrocities of war to the general public - many of whom still saw going off to be a soldier as a brave and romantic endeavor. James Nachtway, who has been hailed as one of the best of the current generation of war photographers, has said many times that "photography can be a powerful ingredient in the antidote of war".
