I think because she clearly balanced the numbers of good vs. bad characters. Now what does fandom do? We are trying to look deeper into her designed-to-be-bad characters and discover that they actually have a good heart, if only one looks deeper. And what happens is that the carefully cultivated balance is gone. (heh I don't think she knows the fics about Harry and co. turning evil are almost as popular as about Snape and Draco getting good roles.) Of course she wouldn't want that, or she would be rid of all her bad characters. (Well, except Voldemort. Hm... needs to do something about that. *G*)