So think about Fluttershy's insults to Rarity and Pinkie Pie for a minute. She insulted their lives' callings. Everypony dreams of the day they find their calling, earning their cutie mark in the process. She berated and belittled that.
I mean, what she did was rude, but if you consider just how much a pony's life calling means to them, then she was indeed a monster.