Hmm. I can see what you're saying - and it's a good point - but it's not really what I mean. Maybe I should have used 'humanity' rather than 'goodness'...

spikes_wish has already pointed out one aspect, and I'd say that considering the number of people who are deeply troubled by the Slayer spell in 'Chosen', a Slayer is not just a girl with superpowers, and the darkness of the demon runs deeper than their own. Buffy *does* get off on killing things, and that is not something natural.