I think some of BtVS's worst problems arose when they put the metaphor ahead of the characterization or the plot. If you posit that vampires have no free will, then they're not really characters at all, just walking allegories. None of their actions or decisions mean anything, and why should we care about them? Once they start becoming individuals with personalities and motivations, they become characters, become real, become people, whether the writer likes it or not, whether the initial framework of metaphor allowed for it or not.

And once that happens, you can continue to treat them like characters, or you can try to stuff them back into the confining box of metaphor. ME tried the latter. Unfortunately, the parts no longer fit very well, and a few limbs got lopped off in the process.