I wouldn't call a setting where mages need to be locked up in towers because they can possibly be possessed by demons and tear apart the land, not to mention where there is a country where magic is practiced in the streets, "low fantasy". "Low fantasy" settings have few to no supernatural elements.
"High fantasy" also doesn't exclude political intrigue and betrayals, Lord of the Rings is an example of this. Forgotten Realms and Dragonlance settings are other examples of staple high fantasy worlds, and both feature plenty of political intrigue and betrayals. The main theme of high fantasy is a grand struggle to save the world, which Dragon Age is all about.
About Morrigan: she only cares about herself and at many points is against helping people, at some points even supporting killing of innocents. That's the very definition of "evil" in a fantasy world.
Whether or not she had a choice into becoming who she became (this is also true for the other characters) doesn't exclude her actions.
