User blog comment:Gcheung28/Submit Questions for Supernatural at SDCC/@comment-12891159-20130729185854/@comment-1509826-20130729190650

Yes thats how real demons are (if you belive in demons) but in the show they hadn't decided to include Angels when they first introduced demons, and after sevearal seasons they needed to come up with an explination for where they came from. So in this universe they're broken human souls. Its simply because at the time the writeres hadn't decided to include angels.