Part of this all must be the role the villain plays. In fact, perhaps the word villain is part of the problem. Take Scrooge for instance. He certainly starts out as a villain, but goes through a character arc that has him come out the hero. He is the protagonist who goes through a transformation. Darth Vader is the same. He is the antagonist who goes through a transformation from bad to good—or actually from good to bad to good when all six movies are considered. He is certainly villainous, but ends up good.
Voldemort fulfills a different function. He is a villain who stays a villain throughout the seven books, and in fact seems to get more villainous as the story progresses. In the back story, it's clear he wasn't always a bad guy (again, I'm basing this on the movies only, since I haven't read the books). I understand he doesn't go through a bad to good transformation, so remains a villain to the end. We hate Voldemort in the end. We love Scrooge in the end. We sort of love Darth Vader in the end, though he has less time to make amends than Scrooge did.
This all brings me back to my beginning point: Is the conventional wisdom, as taught in the writing classes I've attended, correct? Must we give our