LOTS OF SPOILERS AHEAD FOR BOTH!
Darth Vader was not always the most interesting character in the Star Wars series. Sure, it was awesome seeing him force check the crap out of people.
But it was Empire Strikes Back director Ian Kirshner who decided to give Darth Vader a little bit more depth by using a concept given by a Star Wars screenwriter (who was NOT George Lucas) to combine the two characters.
So that brings us to Potions Teacher Severus Snape. I have this theory that J. K. Rowling originally wanted Severus Snape to be a bully to Harry Potter throughout the series as never have any redeeming qualities, like that of the Defense of the Dark Arts teacher in the fourth book's joining of the Dark Lord in future plots.
However unlike that Defense of the Dark Arts teacher, by the time other books were released and the films came out, Rowling decided to make the character more interesting by making him a more sympathetic character: she made him a victim of bullying from the Potter gang in his generation before becoming the unpopular Slytherin biased teacher of the series.
Signs of this include the aforementioned evidence and the fact that there is a lack of grey areas between good and evil the Harry Potter series has. Even if Snape and Draco do redeem themselves, there seemed to be this naive notion that there is good within everyone rather than what really creates someone into becoming an evil being.
Rowling does not really get into the economics of crime, and rather goes campily darker with the notion that some people just want to kill things, mentally ill or not.
Hopefully, future lessons about evil can come from that of Ian Kirshner rather than that of J.K. Rowling.
