Trust me. I'll do my very best
😡
I want to point out that Buffy does not seem to like Tara all that much at this point. I think her family-comment is a gesture to Willow.
Cold-hearted pragmatism isn't really a defining feature of Buffy. The order of knights that hunt Dawn serve as a foil to the Scooby gang, just as the Initiative did in the last season.
You are incredibly hard on Dawn. Buffy is not always the greatest big sister or substitute mum. I do not see why her being a "spoiled brat" matters. Does that make her more deserving of being made a sacrifice.
There is no way in Hell Spike would have voted to kill Dawn. I know he is evil, but that just would not be his style. Spike would rather face Glory head on, like he did the slayers he killed. Furthermore, he loved the little Bit, and when Spike loves someone, an apocalypse isn't going to make him turn on them.
I think it was brave - perhaps foolish - to introduce Dawn so late in the show and have her be at odds with Buffy so often. Still, Dawn was a great character with so many amazing moments. She had great chemistry with the other cast, she was a great dancer and she was always relatable.
This is the girl you want to kill off.