She's aware of what is going on with Giles and she knows why Giles went to Faith. She's tremendously upset about that fact.

I'm not sure I follow you.

Do you mean that Buffy is upset because Giles doesn't see her as a killer? (Sorry if I misunderstood you. It's 1 a.m. in Moscow and I'm already sleepy.)

Because my impression is that Buffy was mostly angry because it was Faith whom Giles has invited to help him. Buffy is so angry that she even can't bring herself to say her name - she talks about "HER!"