A lot of the time that Buffy and Faith spent together also took place after she turned to the dark side so to speak. Faith was confused, scared, angry, jealous, greedy, lonely!! Sure, it was a horrible thing to do, well there were a few horrible things. But others have done far worse, Angel turned and Killed her friends... tormented the people she loved. Spike.. spike tried to .. and was evil before they got together. Couldn't Buffy one day come to terms and understand all these things that Faith did in a time of anger, frustration and disconnection. Even forgive her, like she has her 'Boyfriends' . Yea, I know Spike got a soul, then Angel regained his soul, but they still hurt her after that aswell. Everyone makes mistakes, Faith just tends to make big ones. Buffy will see that, someday! 😉
Were not saying that Buffy is perfect and Faith is the only flawed one, we are just coming at it from Buffy's POV. And all their interaction after Faith turned to the dark side pretty much compromised of them fighting. Faith hit Buffy in all the sore places, she has hurt her moe than I think Faith even knows or Buffy would ever admitt.