That's so annoying. Quit blaming Shin for what other people keep doing. he Pope is the one who claimed all the bullsh**, not Shin. Quit holding it against him, you twits.
@zing there is no reason, technically. But one of the "higher demons" initiated conflict to launch his original country back into the stone age and then just let the Demonkin loose. It's not about whether the humans should fight the demons or not, but a preemptive strike against a perceived enemy force. Those are unfortunately common in wartime, and against classic forces it's kind of rude, maybe - deterring enemy forces is, I believe, usually a bad idea, as it just fuels more resentment on the attacked forces' side.
That's with the usual sides being human and mostly having similar philosophies though.
Here you have classic human psyche, going against an altered human psyche which we could say has some "restrictions", that we could call "morals", removed. In the absence of morals whatsoever (whether active, actively repressed or actively turned against themselves - all three already possible in humans), those human-like deviations' tactics cannot be evaluated by human-based standards (some would say they're savage or feral, but I hesitate), and thus any military response has to be evaluated with that in mind.
tldr: You literally cannot assume that they think like Baseline. If you do, "you'll doom us all".
EDITED TO ADD:
Based on advanced knowledge I saw from other people who read the LN/WN, it seems that Demonkin are not completely incapable of (Human Baseline-aligned) morals. Thus we can't necessarily accuse them of being devoid of (Human Baseline) morals. The theory would then be that Demonkin Baseline's neurochemistry is different from Baseline in ways that affect the moral (as perceived by Human Baseline) evaluation of actions, as being drunk on power usually wears off after a bit.
@Zing They don't. They are fighting because they want to fight. The demons beside Strom want to conquer the world. He see's no reason to stop them and is simply bored for now. Becoming a demon does seem to enhance existing "negative" traits such as sadism though. The aggressive nature of demons may make it impossible for most the simply live without harming the easily abused humans.
This is all new of course so it might be required that they have this fight even if demons end up as part of the world later. Demons clearly have no respect for those weaker than they are. If the humans don't have a way to combat the demons they will be abused forever. Thats how overwhelming power tends to work after all. The one with it gets to do what they want, regardless of what others think. People say power corrupts but the truth is that it does the opposite. Power allows people to be themselves without fear. It just looks like corruption because people hold back so much in everyday life to keep themselves safe. In this case the added aggression from being a demon will only make the true natures more prone to unacceptable levels of violence.
Gotta love some good ol' treason. Just read the latest Black Healer chapter before this and it's a similar thing there, too. Some blockhead in a position of power who won't accept that the protagonist is getting special treatment wants to get rid of them, even though it would lead to the status quo taking a supremely negative nosedive, decides it's time to act against their direct orders and the interests of their superiors. Goddamn. If he's gonna turn demon, just turn early so we can get it over with.
@lionfromnorth I actually didn't read either the WN nor the LN. I'm just theorycrafting logical reasons why there's a preemptive strike force going at the Demonkin in that storyline, to be honest.
Also, to be fair, based on the civilization level illustrated in the manga, that whole world (minus Shin) doesn't have quite the same understanding of war, human psyche and long-term effects of wartime actions as what we'd understand to be Baseline ourselves (and even then, we have Baseline variations throughout Current Earth, depending on the dominant culture and political regime/s in place). So it's just as possible that their dominant version of Baseline still has preemptive strikes as a matter of course, and thus is morally defensible from their point of view.
Biologically speaking, if I had to theorize: depending on the embryo/fetus' development and in-womb strength (also speed of development and physical womb hits), Baseline Human women may be in danger if carrying a demonkin embryo/fetus to term, whereas Demonkin women likely would not have as difficult a time. The biology itself doesn't change much if we look at the depiction; what seems to happen would be strength factor unlocks and neurochemistry alterations based on a VERY different variation of magic/mana flow.