Kazuya literally came out of a couple years coma and fended off a demon with just a pistol.
He literally has
that resides inside of him and gives him power in shape of magic, there's not a single way in which Kazuya can serve as an example of an average human. And that sudden burst of magic was the only reason he didn't get raped to death by the barghest that ambushed him.
And nothing is stopping those strong people from protecting the weak ones and the demons they befriended could help too
Those strong people who befriended demons would just be viewed as brainwashed or parasitised by demons and treated as enemies. You can't convince people that the monstrosities that were going on a murder spree literally yesterday are all good and fluffy now.
You know, like how mother rabbit punches the fuck out of a harpy to protect a child.
No, she didn't, harpies were pointlessly bashing their heads upon the glass of the window. The child was safe, locked in her room. She died after opening the door to the Mother Rabbit. If she didn't open the door, Kazuya might have been able to save her, because the town was invaded only a week before Kazuya's awakening, there's no way she might have died of starvation in a single week.
I doubt genocide is popular because "demon bad, must kill". It's popular because 99% of games don't give you the option to kill everyone in your path and get a unique storyline out of it.
You know, like how mother rabbit punches the fuck out of a harpy to protect a child.
Wrong again. Undertail also has genocide, but there's barely any people who actually prefer this route. People love genocide in Shrift because it's demons are actually evil, their entire being drives them to murder humans and sparing them is actually harmful for humanity, unlike monsters in Undertale who were mostly benign and didn't wish harm to anyone.
Edit: And besides, Hyakki flat out says that all the actual bloodthirsty monsters have already gone ahead to continue the conquest. Those demons would just murder Kazuya without wasting their time on bullshit like trying to rape or brainwash him.