The story of Arya is one of tragedy. We're talking about a child whose seen nearly everyone she knows and loved brutally murdered in the most violent ways possible, and got tossed quite literally ito the dirt with no one around to help:
even moments before she would finally reunite with them.
This was her reaction to hearing someone brag about personally killing her family shortly after:
She can only sleep soundly at night if she explicitly says the names of every person she wants to kill.
And for months to possibly years had to endure explicit torture and brainwashing in order to become an assassin:
This is the face she makes as she gets revenge:
It would make zero sense for Arya to be a sane well adjusted person. The scariest thing is what happens after she's finally "satisfied" with her revenge. She can't settle down or live a normal life.
The waifs biggest mistake was refusing to kill her without making her suffer. I wouldn't be surprised if she purposefully avoided vital organs.