Do you mean that Buffy is upset because Giles doesn't see her as a killer?

No, more that Buffy is upset that there are obvious shenanigans going on that she *knows* she wouldn't approve of. That she knows there are things that Giles would consider doing that he would go to Faith first instead of Buffy.

I think Giles and Buffy are on course for a major blow out where she's going to wonder if it's up to her to stop him.