I think she was being so manipulative because at that point, she felt a kind of jealous hatred towards the Scoobies. Tara still being all shy around everyne at this point, Faith saw the opportunity to do som real damage. Let's not forget she was the sort of girl that felt Buffy and her gang had it so much better than herself, and she didn't deserve to be in her own showes, at this point she still didn't understand that it was her own fault and she had made severe mistakes.
I kinda like the scene for how Faith's character's is coming across. It was clear to see that at that point, she only felt bad for herself and otherwise had no sense of what's rght and wrong. She was purely driven by her own emotionally immature revenge on everyone else.