Imaro
Legend
But this angle actually makes them more simplistic...
You basically have turned them into a human "badguy" in terms of motivation. What makes using a devil different from say a high level spellcaster?
So all the various motivations that a human can have for their actions are more simplistic than the single motivation of... I do evil because I'm evil... seriously? I've never viewed human motivation to be simplistic in any way.
Attach any human motivation, whether sane or mad, to an immortal, perverse, corrupt and alien genius, extrapolate the methods they would use in accomplishing whatever that motivation is and you have a fiend. I don't think what seperates the mortal spellcaster from the fiend is their differing motivations... I think it's in the way said motivation is acted upon by each.