A good call from Frieren to make those two fight on their own. That's the only way to really build their character.
Since the demons disintegrate into nothing but mana particles when dying, it makes sense they are all interested in magic. They don't seem to be that interested in anything else. I reckon they are driven to kill humans using their specialty magic because what good is only theory? They surely must also keep experimenting with their personal magic to develop it further.