Board Thread:Questions and Answers/@comment-5693502-20160425161448/@comment-25942629-20160430225305

Demons are desire-crazy, wanting to kill and rape people all the time (if it's a human becoming a demon--we have yet to see a vampire evolve into a demon). Vampires only desire blood, which makes them act quite a bit more civilized.

Most of the demons we've seen up until now have been cursed within the weaponry invented by Mahiru, who became a demon herself while developing the cursed gear. Guren is slowly becoming a demon as well. I imagine he's only transforming so slowly because he still has Mahiru-no-Yu restrained by curses. Yu himself is becoming a demon now, so we should see more of how newly-born demons non-cursed demons behave in their natural form.

Vampires eventually become demons, if they are powerful enough. Kureto becomes annoyed in the manga when he mentions that he can only force a few of them to turn into demons, and, even then, they make weaker cursed gear than he wants. This is the reason the JIDA wants to capture nobles so much. I suspect Krul may be relatively close to becoming one because she appears to have a pair of small black horns on her forehead, but the art makes it look like they could also be hair clips in her bangs. When she thrashes Ferid in the manga, there are also dark flames coming out of her clothing. This was left out of the anime.