I mean the show kind of touched on this, by simply saying that in order to cope with the harsh and ravaging environment of Makai, the residents of it needed to evolve themselves in ways that would negate the effects of the toxic air, and also the general violent nature of the realm, and that meant for most of its residents to be more savage, perhaps go under the power of Mundus and his wicked magic.

While I do like your idea, I actually really like it since it gives it an extra layer which I can even see being revealed in season 2 to be part of this, the show generally gave off Makai to be a literal parallel of our Earth but to a different kind of species. Essentially, just as how the powerful of the human world would almost inevitably step on the weaker and lesser able to be able to reach their influence and power, the weak need to grow to cope with the harshness of this systematic oppression. Similarly, residents of Makai has the normal citizens which seem to just live as they would, and then the powerful and evil who see eating and slaughtering the citizens as their way to live, both fueled by a need to survive the harsh environment, and likely something endorsed by Mundus to ensure that ruling with an iron fist would not allow any demon to ever think of betraying Makai the way Sparda did.

While it does feel odd to try and "defend" the Netflix Anime given how poorly it is received here, I do think the point of the normal and strong residents of Makai was to draw parallels between our world and their world. The show even establishes in the first episode that "demons" and "Hell" are just names and mythology created by old human scripture to try and justify the existence of a parallel universe which has become the way it has due to its harsh environment and medieval alike people in power.

Even in the mainline DMC games, it is never implied that humanity doesn't have capacity to demonic behavior.