That and the reason the war began in the first place. The first chapter basically just says "yeah, they just showed up out of nowhere and just started murdering people," which doesn't actually explain anything. Plus, "they sudden attacked us for no reason" gives off huge "its our fault, but these are our history textbooks so we're going to pretend its not" vibes. (You know, like American colonists and indigenous Americans.) Plus, the intro claims that humans and demons never interacted before the war, but, assuming the boy in Lieselotte's flashback on p30 is Schmerz, (which, like, come on) that's obviously not true.
We also see a consistent disdain for demons coming from humans, and while we haven't seen very many demons as of yet, the ones we have don't seem to reciprocate that. Lieselotte's in love with a human, of course, but presumably if she did come from a human-hating culture, she should still have some deep-set biases, you know? And her maid doesn't seem to have anything like that at all. To be fair, her "open-mindedness" could be the reason she's the one here rather than some other maid, but again she should still have some internalized biases if she's coming from a culture that widely hates humans. The closest she gets to this is saying that "fear is all it take a human to surrender," but this is clearly a point of demon pride rather than perceived human weakness.
Another point against demons just being inherently evil murderers is the fact that it was the demons that proposed peace and that they were able to get their public to support this. Plus, it was a demon that fell in love with a human.
So while its not like this is 100% definitely what the author's going for, I think that there's a very real possibility that the humans were actually he ones that started the war in the first place, with the painting of demons as "evil" being a form of propaganda to justify it.
And "demons are actually misunderstood" is like a whole trope and probably not especially strange to expect in a series about a romance between a human and a demon.
