The animosity between Arianna and Flame Thrower won't just go away because it's revealed that Dispawn truly was evil. Sometimes the human mind is irrational, and when you don't want to believe something, you do your best to create a new version of reality in your head. The thing is, Arianna doesn't know psychology or the like. She's used to getting her way with telepathy, yet she's reluctant to do it with Flame Thrower. Thing is, she's still scared of her, and so feels she needs to assert her dominance.