I think what made it so chilling was that fact he was so calm about it, as Willow My Life said. But that was the point. Giles' is very caring and is such a gentle soul. But just as he said, he has a duty to protect this world at any cost. And he meant it, even if it meant killing a another person. And Giles' was able to make the choice without hesitation, and so coldly.
I mean. If it had gotten down to it, Giles would have killed Dawn to save the world. I really think he would've of. It would have hurt him immensely, but if it meant closing the portal, he would have tried to kill Dawn as quickly and as painlessly as possible.
