Then I won't spoil you unless you explicitly ask me to. But yeah, I would say the idea probably occurred to Joss somewhere in the middle of Buffy, but by that time he'd made Giles too "safe" to turn him into an outright villain so he just filed it away for future reference. Yes, Giles gets to be dark occasionally, but it's done in a way that usually makes him more heroic ("Look at what he's overcome!"), showing it so we don't have to wonder about it taking him over. By and large, Buffy never questions that Giles is - or, given the chance, will be - the Good kind of authority. Which makes the idea of a villainous Giles all the more interesting.