Hah! yes, I see where that would be exactly the problem with S. and a number of other more-or-less white hats in the Potterverse (at least in fanfic.) One of the best ways to motivate bad (selfish, unthinking, greedy, etc.) behavior in an ambiguous character is to have them run up against material limitations imposed by the real world. In a magical world, where there's likely to be more than enough for all, you lose that possibility - it's too easy for Scrooge to act like Santa, and there goes the story.

Somehow this is all connected in my mind with the Mary Sue issue but I can't quite figure out how.