I wish demons were more varied in their morality.

I think this is because of how the concepts of demons and spirits are settled up. They were ''the first children of the Maker'', creatures made to exalt the work of the Maker and create their own path within it, something which ultimately they couldn't do. The Chantry myth says the Maker created mankind because of the disappointment he experimented with spirits. Demons are then rebels to the Maker's purpose, the same as in the Christian mythology, who lurk into mortal men dreams and souls because they can't hold back their yearning for experimenting human creativity and emotionality.

The whole concept of the Fade is the key here. I think it is a very creative way to reference the world of ideas and how they become a real force when they are assumed <<in reality>>. We are told by Solas that the Fade can be molded by our own volition, by our own perceptions, something that will have its implications in the <<real>> world. Demons and spirits seem to be in continuity with this: ideas and emotions, by themselves, are determined in their own character. The only way they ''evolve'' is by disappearing into something new. A demon or a spirit does not know any other way to behave than... its own.

I suppose there are exceptions to this (maybe and specially with demons due to their closer contact to human creativity) but this is what I read from what I know about the Fade and its inhabitants