I thought Twilight was using some sort of mojo to make the population anti-Slayer (which I guess could mean pro-vampire). Besides, Harmony's kill could be spun as self-defense, which would provide enough cover to make a sympathetic audience go with it. (And being reality TV, it was probably edited in a highly manipulative way.)
It's not my favorite plot point either, but I think it can sort of be rationalized within the story.