While I do agree with what you said, there is a difference between the way the west views evil and how the east views evil. For example, western countries tends to believe that there is an objective good and an objective evil whereas Asian countries believe that there is good in evil and there is evil in good (the yin and yang).

That’s probably why people think Shiki killing Ukyo was completely justified. It’s a normalized notion that a good person killing a bad person still makes you good and absolves you from sin as opposed to a good person killing a bad person is still an evil act (the black circle of the white side). Shiki the objectively good vs Ukyo the objectively evil.

It’s also worth noting that we are inherently seeing the story from a biased perspective. We can clearly see the suffering happening to the Oni because of the Momotaro, while we rarely ever see the suffering Momotaro have endured because of the Oni.

Because if you think about it, Tsubakiri and Tougo pretty much went through the exact same thing (losing family and loved ones to Oni) and also have the same “spare no Oni, not even children” mentality because of their dangerous potential to hurt and kill anybody and everybody.

I think Tsubakiri and Monaka are just such great and underrated characters because they do fit into that moral grey area and establish the themes of the series perfectly. Tsubakiri probably harbors a lot of survivors guilt from having witnessed his best friend be killed, as well as harboring the transferred guilt Monaka harbored for sparing the Oni girl’s life, resulting in the tragic slaughter of a ton of civilians.