Well, unless you're doing a purposely AU thing in which the bad guys are the heroes of their own story I guess
But we already have a trope/tradition for that in western storytelling: the anti-hero. (Which has become a meaningless term at this point anyway.)
I'm not at home right now so I don't have the link to Foz Meadows' brilliant meta on Orphan Black and the female anti-hero (which explores this trope in terms of gender and briefly mentions Buffy and Spike in the context.) brilliant, and should be required reading.
Re: I have to rant I HAVE FEELINGS
But we already have a trope/tradition for that in western storytelling: the anti-hero. (Which has become a meaningless term at this point anyway.)
I'm not at home right now so I don't have the link to Foz Meadows' brilliant meta on Orphan Black and the female anti-hero (which explores this trope in terms of gender and briefly mentions Buffy and Spike in the context.) brilliant, and should be required reading.