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Would What the Hell, Hero? fit what you have in mind?
It's that line of thinking. Angel doesn't think what the hell Buff, he knows exactly why she's acting the way she does, knowing about evil Faith did that Buffy probably doesn't. It's more a case of Angel not believing she has a right to be upset, or if not a right to be upset then to kill Faith.
It's kind of a double WTHH moment. For Buffy, why should Angel shelter Faith and try and redeem her? For Angel, what right does Buffy have to kill someone?
Could it be the whole Slayers don't kill thing? Or maybe Buffy still fighting the dark side that Xander noted when she was set to kill Faith before?
I put it there, is there something from perhaps Buffy's point of view (she hurt me, I want to kill her) that might also work?
That's actually part of the criticism over Buffy's actions towards faith. She kills the deputy mayor, holds Willow hostage and threatens to kill her, she kills some random scientist, and sure she's evil but Buffy takes it in her stride. Angel likely never said anything about what she did to Xander, but when Faith poisons Angel, suddenly Buffy's all for killing her, going so far as to ignore Xander's concerns that she will become Faith. What Faith did to Buffy is her justification to ignore Angel's bid to reform a repentant Faith, wanting instead to simply kill her. I put this under that trope.
Is there a trope for Buffy, the hero, going on Angel's show to kill Faith, a reformed bad guy, who the show's main good guy defends to the point of Buffy coming across as the villain this time?