Even with the reveal that there's a typical evil religion behind the war, it still makes the conversation between the hero + demon king kind of goofy. Like, if they were aware they were being manipulated into it by the religion/s, it seems strange that they would have had arguments about why the war was started, but if they weren't aware they were manipulated, "humans started the war because we decided monsters were too strong and had to be put down" is still a really shitty reason for someone to wage war against a friend + his innocent population, lmao.