This isn’t a baby hitler dilemma at all. This is the equivalent of shooting hitler the moment he gets up to speak at the beer hall putsch. Mielle confessed, unprompted and without duress, that she had been plotting for aria to die, alone and in pain, from the very first moment they met. We are told in the very first chapter that aria began tormenting mielle on the suggestion of her maids, who were controlled by mielle, by her own admission. Aria was a brat definitely, but mielle is the first to escalate to lethal conspiracy.