@Add1152000 It's a bit of a trope, but I think this manga follows what a lot of other dungeon master reversed stories are doing...that being that demons aren't active antagonists.
Oh sure, they kill anyone who who enters their territory, and the demon lord apparently has a roaming party that goes out to fight heroes...but basically there doesn't seem to be random acts of violence here. They're protecting the tower essentially, and there's probably something integral to the world up there. The humans as a whole know the 'rules', they enter the demon's domains, they can expect a fight to the death. It's just fair in that as long as its a human, doesn't matter if its a hero or a kid.
Adventurers and dungeon monsters both seem to garner employment from this...which as a system isn't actually unprecedented. Flower Wars instigated by the Aztecs were formalized wars where each side literally would kill each other for training, or take captives to use them as sacrifice, and served as a bedrock of society's functions. They even had laws regarding employment, which extended to warriors, even in this context. For less formal and more chaotic examples, levied armies in the 100 Years War in France was, at the solider level, simply an opportunity to move up the equivalent of the "corporate" ladder at the time once they got back home.
It only doesn't work if you simply cannot accept that killing can be a respectable and societal approved profession.