Recently I've been learning a lot about personality disorders and the like in a psychology class. What she is saying is so, sadly, incredibly true. Its easy to think of someone as evil and move on, but the lasting impression goes with the ones who makes us think, reflect, and feel bad for them, despite their evil ways.

ah yes. This is something I try to express to a lot of writers as well. Villains aren't always evil people and often are result of bad choices, consequences of actions, and trying to form a new path in just the wrong way. As well as a hero is in the eye of the beholder. A villain could be a hero while a "hero" could be a villain.

This is something I question often with supervillain/villain when I watch shows and movies. It's easy to just write a bad guy character evil simply because they are...but it's always a more entertaining a gripping story to find out WHY? Even with the Joker in The Dark Knight we know he is strictly a physical embodiment/metaphor for total unadulterated Chaos...but how did this version of Joker come to this philosophy of life? Sometimes the bad guys just want to watch the world burn...that in itself is frightening enough, but still why?
Anyways; I already take this lesson to heart and even trying to come up with meaningful reasons for certain characters in stories I write (or will write one day) to why (for example) someone has agreed to follow a satanic-esqe cult and go as far as undo reality for his own vision? Or why does a monster character pester the good guys, want their power and want to use it rid all humans from existence?
...and my mind fried. Sorry for a lack of a conclusion.