@Ironclad I think I could help you with that.
The bad guys motivation for being bad is.. I guess the typical bad guys thing, like ruling over the world/human race of the sort because demon race is more powerful and superior.
Since well obviously the human race doesn't want that, they retaliate and eliminate the demon race almost to the brink of extinction.. that was in the past before our protagonist reincarnated.
Now in the present as you probably notice when reading, the human race now clearly has regressed on knowledge, magic and other stuff for some reason.
The reasoning for the human race being regressed it's because the demon secretly influencing them so the human race became very weak and the demon could rule over them with ease, now that the humans are weak.
About the demon get slaughtered.. well, they came in the city and being like..
"Bitch! We're gonna take over your town! Now die you pathetic humans lot!"
basically invading asking for war, and of course.. the humans doing the obvious thing defend themselves.
Too bad for the demon they didn't realize the humans got an ace of their sleeves, which is the protagonist. So in self defense the protagonist who is from the human race, slaughter the invaders.
He can't exactly do anything dude, he's just have quite abundant of knowledge and experience from his past life, which makes him absurdly powerful even though he's still young in.
They're basically humans.. well, not wrong there.. clearly the demons got humanoid form but their not exactly a human now aren't they?
Well that is all I guess, hope it helps!