If you mean Lily, it has to do with her mother if I remember correctly. Apparently her mother was from the country that got wiped off the map when the war with the demons started. That country was full of fanatical believers in the Goddess, so when they all died without help (because Goddess needed her beauty sleep) Lily felt that the Goddess betrayed them. As to how exactly that connects to hating hyumans too.... Oh damn... I can't remember... I think it's connected but I can't remember how, sorry....
I don't know who's the idiot that told the author to make these changes, but many scenes play a lot better in the originanovel than this. Where the heck did that anvil even come from?
You have this really, really wrong. Trashmoki is really bad and he'll get so much worse. However...
1. Hibiki is not a bad person, although I can't agree with her views. She is a legit hero - at least from the perspective of hyumans. And she doesn't love the discrimination, but doesn't think she can really do much about it in the middle of a war. Although she does (or will) genuinely hate the demon race for entirely understandable reasons. In the same way the demons hate hyumans for entirely understandable reasons.
2. The MC is not the flawless "good guy" you think he is. On the contrary he's straddles the line between retaining his humanity and becoming a monster.
In one (unrealized) possible alternate reality timeline that get shown to him by a god, he turns an entire nation into sand in an instant... countless death and total destruction for no particularly good reason. Even Trashmoki was horrified - and perhaps that reaction was his only reason for doing so. In another one he creates a spell that imitates a microwave, and microwaves a person alive in front of someone else (though I forgot who it was).
The world should be grateful he met Emma and all the others, because there is a darkness in him just waiting to come out. And yet he does a considerable amount of helping others too.
3. Those things aside, he will absolutely become
someone who feels nothing at all when killing in brutal ways... not even changing his expression while crushing a girl's head and blowing a hole through another one's chest. Girls he knew where brainwashed and thus not even following their own will.
But before that...
it is about time to create a new lake while killing thousands in the process.
No they really aren't. Although overall they've been pretty heavily brainwashed by the shitty Goddess of this world, there are plenty of decent people. And there are hyumans who aren't especially good nor evil. They're just people. Naturally the really bad ones stick out more.
The hyumans aren't evil. Neither are the Demon Race, but the definitely have bad blood between them. You'll see in time the series doesn't apply the usual good/evil tropes. Nor would I classify that goddess as evil - rather than an idiot who is causing a lot of harm.