It also doesn't help how they're all wearing the same outfits. This manga is like shounen at its laziest. Every villain looks the same, every villain acts the same, and they're so ridiculously monstrous that any notion of nuance or subtlety is thrown out the window and set on fire. It's "good guys vs. bad guys" boiled down to its core components for maximum efficiency. Even the power system is kind of dull. Both sides use basically the same power of "exude substance from body that takes shape or has other random ability that follows no real rules".
You are just plain wrong and are only looking at this manga at a shallow level. The Momo aren't just plain bad guys. In some ways, they are the victims of the bloodline they are born into. They were indoctrinated since birth that Oni are dangerous and needed to be exterminated. Any Momo expressed sympathy toward the Oni got demoted or dispose like MC's father, Mikado and Senritsu. You shouldn't forget that in the eyes of society, the Momo are the good guys, just like in the original folktale "Momotaro". The Momo truly think that they are the good guys, protecting humans from the monstrous Oni. BTW, if you understood Japanese culture, you would know that Oni are beings of ill omens, diseases and disasters. Japanese fear them to the point that they have a tradition, on the day of Setsubun festival, they threw beans (which are supposed to have magical properties) to chase away the Oni and bad luck. And the villains each has different personality and motivation. I don't see how they are the same. I even have some sympathy for Tsubakiri while Kusabi is just pure trash.
Also, this is one of the rare shounen manga that touched on topic about the morality of killing. You can see that Shiki got PTSD after killing for the first time. Other manga just don't let the MC kill anyone or just ignore the issue completely like One Piece, Bleach, Naruto or JJK.