At the end of s5 it wasn't about anyone in particular, so no Dawn hate, just sacrificing one for the well-being of many.
And that was exactly the point, Giles considered that option because it could've saved so many thousand lives. Later in s6, however, there was no saving others by Dawn's death anymore, so her dying would've been very pointless and not what the Watcher's Council stands for.
Btw, as a good comparison. This is pretty much the same thing that was going on in Angel s5 with Fred. Angel and Spike got the possibility of saving Fred after all, but only at the risk of harming so many others. Here Angel made the choice to let Fred die to save others.
Of course the cases here are still pretty different. Giles killing Dawn would've meant killing someone innocent, but by that the inevitable would've stopped. With Fred it wasn't about actively killing anyone, just letting someone innocent die without intervention because otherwise the consequences would've been much worse.