@TNT261 I disagree. On the contrary, I'm kind of tired of how demons are in most manga these days: They are basically just humans with horns or some other decorations and often strong magic. In RL demons are considered the servants of evil, trying to corrupt and harm humanity (or in other words, test the faith of believers and punish evildoers). However, many authors have pretty big troubles trying to depict evil at all. You could consider a real demon something like the successful killer psychopaths in some of the works of that genre. So, they indeed are supposed to be utterly evil, but it would also depend on the skills of the author how nefarious they can make the evil in their work. That being said, lots of manga is quite fluffy, so you can't have really evil beings in it. So, demons are simply made another race. Just like elves in most manga are nothing but humans with pointed ears and perhaps a longer lifespan. Not that different from what happened to vampires in Twilight and other cheap urban fiction.
@ShinGetsu Those who do, and are too corrupted for it, probably aren't going to fight for humanity's sake anyway. However, someone like Zagan doesn't have much to do with demons, as far as we know. I imagine there are more people like him around if the plot requires any army to have magicians in significant numbers. If this story is going to be intelligent, it will naturally also have demons betraying mages who think they are controlling the demons. Maybe the early history of all mages had something to do with demons. Like the demons taught humans how to use magic, but it was only for the reason of a grand betrayal later. Nevertheless, if you are going to fight demons, being called a demon lord is not convenient.