Black and White

In the early seasons, everyone in the Buffyverse was very convinced that everything was black and white. Vampires were evil. There was no room for dispute. Demons were all evil. 

Xander: I don't like vampires. I'm gonna take a stand and say they're not good.

Of course, as time continued, this changed. Angel played a big part in this, as the first ensouled vampire the Scoobies encountered. Granted, not everyone was as open to accepting Angel. But by the time the Scoobies graduated high school, things were definitely seen as being shades of grey and not as black and white. 
Heck, by the time that we get to season seven, the group often get assisted by demons. (Spike and Clem specifically). Even Xander's black and white ideals changed. He went from the boy who hated anything not-human to the man who fell in love with a (former) vengeance demon.
The shades of grey is a recurring theme throughout the series. It's one of the biggest sticking points in the dispute between Buffy and the Watcher's Council. By the time she turned 18, Buffy had come to learn that things are not as black and white as everyone would  like them to be. 
For someone who has actually grown up with the show (I was seven during the first season), this changing viewpoint of what it is to be evil became a vital part of how I see the world. I spent a great deal of my formative years alongside Buffy and the Scoobies.
Is someone evil because of what they are? No. They are evil because of what they DO.
As a child, I was always told that actions speak louder than words, a lesson I have carried with me. In life, things can't always be black and white, but it's the shades of grey that make us all unique anyway. (Remember my blog post about respecting our differences?) Instead of fearing the shades of grey that surrounded their everyday lives, and trying to live in a fantasy world where everything was either good or evil, Buffy the Vampire Slayer showed that nothing is black and white. And it's yet another on the LONG list of reasons why I am very grateful to Joss Whedon, the writers, and the cast.
~Buffy