Based on just that last page, it's almost 100% the humans fault for everything. I bet they tried expanding into the demon land or started slaughtering their people for being different but claimed it was the other side. It's also not like the "hero" has done any good for demons either. While there were most likely humans killed by the princess, who knows how many demons were slaughtered by the hero personally. I'm also betting with his town, it'll come out that the humans did it