From the Slayers perspective, Buffy LITERALLY slept with the enemy. They knew nothing of the history between Buffy and Angel, all they knew was that Angel was Twilight and the one responsible for the war they found themselves in the middle of, he was responsible for causing the deaths of their friends and sisters.
From Kennedy's POV, this was Buffy once again choosing a vampire over her and the rest of the Potentials/Slayers, only this time, Kennedy had her own power and she could take that power and do what she wanted with it.