Because, you know, not everyone has that odd photographic/sievelike memory combo thing I seem to be stuck with these days:
GILES: Can you move?
BEN: Need... a minute. She could have killed me.
GILES: No she couldn't. Never. And sooner or later, Glory will re-emerge and make Buffy pay for that mercy, and the world with her. Buffy even knows that, and still she couldn't take a human life. She's a hero, you see. She's not like us.
(Giles suffocates Ben)
From Buffy Dialogue Database: The Gift, if that wasn't clear before.
Oh, and although this should be obvious?
I'm not a hero either.