War crimes and the alike are a more common thing in traditional publishing. Of course, especially historic fiction.
The simple reason is: the majority of readers don't want to read about war crimes. That stuff is horrific to no end and simply doesn't fit the mood of most webnovels which are geared a lot more towards "brain off and enjoy". And what would you use war crimes for? For a cheap shock in 2 chapters? To show, how comically evil your villain is? Or to make a gray world where there is no good?
The simple answer is, war crimes are way above that level and it'll make that stuff so over the top bad, people won't take it serious if you just throw it in. War crimes are such, that a lot of people can't even imagine the actual extent that happened during WW2. Some people don't even know how to write/read the numbers/chemicals/procedures associated with those crimes. So, if you don't be really careful and build it up through your entire book and set the atmosphere right beforehand, it is just a cheap turn-off for most readers.
War crimes are a bit like rape in fiction. 99.9% of the depictions are shit. Most hobby authors don't even understand what such stuff will do to people who do it, who experience it, or who simply watch it. Most authors don't understand the reasons behind acting in certain ways or why certain behavior is harder than others. Instead, it goes like "I have a bad guy who is really, really bad... so let's make him do the worst thing I can imagine! War crimes!".
So yeah, the majority of authors - and I include myself in that - have never done the research to write such things in a way that is both respectful, realistic, and to some degree still entertaining to read. Which is the reason, most authors simply don't do it... they know, they would just fuck up their own novel.
If you want to try it: Good luck with that.
But be aware, that there is a lot of work to be done besides "I want cool dark stuff in my novel!"
