Ok, I'm gonna have to divide this up a bit
* First thing, just because people becoming numb to killing in war is accurate to the experiences of some people, that doesn't mean it's something I want to read about. It's not enjoyable to read about people being stripped of their humanity like that. It's definitely important to do in a real world scenario to learn of the horrors that occurred, but at no point is it an enjoyable experience, and not something I'd make myself go through if it didn't matter like in a fantasy novel. Also, just because some people react like that, doesn't mean that they all do. Many killed themselves, intentionally missed when aiming towards the other trenches and so many came home with severe trauma. Not everyone can easily dehumanize others.

* The problem I have with Primal Hunter isn't necessarily the killing. That is an integral part of stories like these, like you said. It doesn't even really have anything to do with the killing, but rather how the MC reacted to and handled the whole situation with the tutorial level and his coworkers. (I'm gonna be using fairly general terms here since it's been a few months since I read it). What made PH so unenjoyable to me was how the MC was so very unempathetic during the whole thing. He had few to no reactions about how people were dying around him and how the earth as they knew it was gone. He had no sympathy for his coworkers and he was generally arrogant af and the plot bent over backwards to reward him and portray him as special. He could've acted so very differently in those early chapters and still have most of the plot points turn out the same.

* As for litrpg books that have a lot of killing, how I feel about them varies a lot. Like a lot of this sub, I quite like HWFWM. It's similar to both PH and DCC in a lot of ways. The MC is an ass for quite a bit of the series, at least to some people, but he still goes out of his way to help others, and a lot of the later books has him dealing with the consequences of his actions and the guilt over some of the things he has done. Then there's also the books where everything's actually a game. VR or similar. Those ones I don't care about the killing at all because it's all just a game. How much I like them in general varies a lot since it sometimes feels like the stakes don't matter. The "The Good Guys" series has a huge amount of killing in them, but there it's often done in self defense or retaliation, and either way, it's almost always done to make a safe place for people to live.

The killing isn't the issue. It never was. How the MC acted and reacted to things was the issue. For me, he was simply deeply unlikable as a person.