Consider the relationship between Spike and Giles and quite why it was always so antagonistic. Now, slasher interpretations aside, you would expect them to be a little rubbing each other up the wrong way - Spike after all does delight to tease Giles about his status and rub the fact that he is in fact so much older in Giles' face. Plus I suspect Giles actually makes Spike feel a little insecure, re-raising all those ugly class based issues he has with his own people. (Probably also the reason he was so cool towards Wes in AtS 5)
Anyway, my big revelation was about why Giles might hate Spike in return. because it is a slightly odd fact that he does, and one that I feel isn't entirely covered either by the fact that Spike is a vamp or by Giles' feeling of fatherly protection towards Buffy in the face of an unsuitable boyfriend.
The thing is it may all date right back to Season 2, because Giles may not actually know the role that Spike played in the Acathla business. After all, Buffy never talked even to Willow about that day as we learnt in Empty Places. So, as far as Giles is concerned, Spike was simply one of those present, laughing and egging Angelus on at his torture. But worst of all, Spike was the one who suggested the ruse that eventually broke Giles. Giles has to hate him for that. Hate him unbearably for having caused him to give it up when he had resisted Angelus for hours.
As I say, the rest of you probably realised this years ago.