If it was like that, people in Thedas probably wouldn't be such uptight little shits about it, but it isn't. I don't really know all the mumbo jumbo jargon, but I think they draw their power from The Fade, which is just stupid complicated to get into. If mages go bad they turn into abomination demon things FROM the Fade, or so I hear. Never met one personally.
That's funny you should mention that,sweet thing, because Ferelden actually HAS a folk tale about a witch that lives in the wilds and scares everyone away.