Okay -- so "mazoku" ( 魔族 ) = "magical family" or "magical beings" (= "creatures of mana", as proposed by -D-, which is why they disappear in sparkles?) I take it this is a made-up word for the series and not a traditional Japanese yokai? While the words themselves don't seem to have any indication of hostility, the fact that they all have horns implies the writer seems to be taking at least a bit of inspiration from western demons -- although the horns aren't strictly necessary. As we've seen, some creatures like spiegel don't have horns at all. And if we say spiegel is a "magical being", that is definitely something no one would argue with. I also have to wonder if all magical beings are necessarily as hostile as the ones like the Demon King or Aura the Guillotine, who absolutely were inimical to both humans and elves and deserved what happened to them.
ETA: It also ends the argument "Is Spiegel a demon or not?" Because the word, properly translated, means the question is better asked "Is Spiegel a magical being or not?" And the answer is clearly yes. No dispute possible any longer.
Respectfully,
Brian P.