Yes, Faith was horrible to Buffy, and yes, she did some terrible things, but Buffy needs to be able to suck it up and get over it. She is the slayer: she needs to see the big picture. Angel was always seeing the big picture, and Buffy always knew that he would try to help her. It's just that Buffy has always been jealous of Faith, and that severely clouds her judgement. I mean, Willow tried to kill her sister, but because they were friends, Buffy didn't consider killing her. It's the same with Anya, she goes all demony again and because Buffy doesn't like her as much, she grabs the sword.
Buffy always holds people to a double standard, and her jealousy of Faith leads to her refusing to listen to Angel. She has every right to be angry when she walked in, but hitting him, then being surprised when he hit back? She had no right. If she refuses to set aside her petty jealousies and see the situation as it is, Angel is perfectly justified in shouting at her. The situation was far too dangerous for Buffy to be blundering her way through with all her emotional baggage. Faith, a renegade slayer, was on the brink of redemption, and Buffy refuses to see it. She knows that the world is not black and white, but for her at that moment it was all green. It's understandible, but she was still very much in the wrong.
I also want to point out that although I don't like what she said, I like that she said it. It was a good episode. And I like Faith too. Mainly because you always knew that she really, deep down, craved to be good, and to be like Buffy. She was jealous too, but she got over it. Well, quicker than Buffy anyway.