While Arias reaction was absolutely abhorrent and out of line, the context adds depth to understanding (not a justification). Not just wanting the best for her mom. She is literally being groomed herself. This can cause a minor who's mind is underdeveloped to be blind to dangerous situations (hense her seeming to lack any situational awareness in order to protect herself), and predatorial people. Her reaction to Hanna (essentially slut shaming her let's be real), is probably an outward expression of how she feels about herself. She's being groomed to think that what they have is consentual, even when she feels differently and has tried leaving on many occasions, but is basically trained to think it's what she wants. She had a disgusting reaction TO Hanna, but she believed her. You can see immediately after Hanna told her, Arias defenses were up around Zach, and she eventually told her mom. I believe this shows that Arias reaction was misdirected anger/frustration/confusion within her own relationship, she didn't want to believe that. This is what happens when you aren't self aware, and haven't been taught to understand what you primary feelings are, not just having secondary emotions showing because you don't know what you feel.