"Seeing Red" is actually one of my favorite Buffy eps, because it has some gems of story telling. Willow and Tara getting back together and then being torn apart, jackass!Xander (though in my book he's pretty much always a jackass), and Spike finally snapping.

Buffy's reaction to Spike's attack was the one of the things that felt off. It felt as though the author's agenda took precedent over the character's story and past conduct. There was a lot of that in season 6 (see the magic=drug addiction story line).