The slayer who thinks he slays is slain.
Before the world was made there were two factions of spirits in a state of chaos. You will find this in all of the ancient creation myths. One faction was in some way divinely inspired and beneficent, and they began the act of creation. The other was dissonant for one reason or another, and the supposedly good faction rejected or strictly limited the influence of the dark one.
As the act of creation started some of the more worldly spirits took up residence on the world, living in it and protecting it. These were the first elementals and devas, the keepers of the balance. But the dark faction was not eliminated even though they could claim no real territory in the world, or only limited territory, and they would at times rage against the world itself. It was believed that if people offended the guardians in some systems that they would not restrict the demons from their protected place.
The difference between elementals and demons is that the elementals were a part of the process of taking form, orderly if not human, and the demons still dwelled in a state of chaos to some degree so they didn’t resemble anything properly in the mortal world, not elements, or animals, or men.
Your thoughts are welcome. Be well friends.