Inspired by ms_scarletibis
's post on the moral standing of vampires
and the human Buffy characters, I thought I'd post my own thoughts on the subject.
First off I see both Buffyverse vampires and humans as species that are very different from real humans. Buffyverse humans have souls and I don't really know how I would translate that concept into reality, where I have not the faintest idea if there's such a thing as a soul and even less how it would work exactly.
The definition of good an evil in the Buffyverse is shaped by two things:
There's the metaphorical plane on which the demons stand for various conditions that ail us in real life, fears, addiction, cruelty, inability to change and people who drain others emotionally...the list goes on. All of those things are bad and on this plane the vampires are by definition bad.
The second one is the in game fictional plane on which vampires are characters with different groundrules. To hunt and kill is in their nature. To ask them to stop is like asking a cat to go vegetarian. They also lack the Buffyverse moral compass, the soul. Inside the verse the line of good and evil is drawn along the soul concept. Those who have one are good or at least redeemable, those who have non are not only evil, they are not people to the Buffyverse humans.
To me from the outside it seems that there are two species at war both of witch don't see the life of the other as anything of worth. It is a war for survival so just the definition of killing = evil becomes too little for me to go by. My view on them becomes defined not by them fighting and killing each other but by HOW they are fighting the war.
The characters show real goodness in my opinion when they show mercy, love, compassion beyond what their morals require them to and their nature tells them to. Buffy could have killed Spike in S4, but it would have been shooting a tiger in a cage. That she did not kill him, though it would not have left a stain on her west defined her as good to me.
Spike throwing his moral set up over board because he's in love with her, did the same thing for me in S5/S6.
So in the end I think good and evil start for me were the basic ground rules end.