I think that Faith wanted Buffy to end the whole sage and I don't think she really cared how, but I think that B stabbing her was a def shock to Faith.
Faith always saw B as the righteous shining light of the slayer line and wanted to be saved, and finally realized that Buffy had given up on her, so she threw herself off the roof as a final act of rebellion, as if to say, you gave up on me, well for that your honey dies.
I also think Buffy was wrong to try and kill Faith, Buffy has always blabbered on about how (and she even preached this to Faith) that being a slayer isn't the same as being a killer, and then the second Angel needs the blood of a slayer, off B trots to kill Faith but its ok because Angel needs it and Faith is 'bad'.