im not sure about what happens after this as i have not seen the anime, however, one thing i will note is everyone in the setting including the hero herself has treated the opposition between the hero and the demons as a forgone conclusion, (...)
Well, that's true, but that's also because of the culture. And we'll get back to this when we get to the problem I have later on.
A world that has been at war for as long as their history goes, with stories of heroes and demon lords feeding their childhood.
But she hasn't done anything except defend herself against a few demons at the school entrance ceremony. Which could justify her going after demons, I might note, but she hasn't done that yet. There is time to change her mind. If anything, the hero is not the most present danger to the demon as the demon leader at that time was killed by someone else. A certain reincarnated onmyouji. Although he didn't advertise his feat, everybody else could testify that miss Hero only killed the small-fry demons.
But there is no "programming" or "forced fate" that makes the hero kill demons and the demon lord kill humans... nor the hero and demon lord kill each other. It's all cultural. And these demons are attempting an assassination, which would definitely justify retaliation. And that's another point we'll get to soon enough.
i think i remember one setting where a hero tried not murdering demons and it just got them murdered by the humans
This would make more sense than what is coming. What you describe is basically the humans seeing the hero as a failure, or even a traitor, and retaliating in "justified" anger.
What's coming here is... so full of holes that I'm having a hard time not spoiling it in a frustrated rant.
For now, let's enjoy the next few chapters as this current section is actually pretty good.