Arya started playing the “game of faces” as soon as she entered the room
The game consists in making a lie sound like a truth.
Arya’s lie became obvious the moment she said she wondered what it would feel like to “wear those pretty dresses” and “be the lady of Winterfell”. Arya would never do such as wearing dresses and she has never before expressed any wish to rule over anything, not even her home.
The moment Arya handed her sister the blade (pointed towards her stomach and not Sansa’s) the game was over and she hinted that what she said (the “threat”) was a lie. ARYA WOULD NEVER KILL SANSA.
Arya did this not because she “despises” or fully mistrusts Sansa, but because she doesn’t trust LF. SHE IS WELL AWARE THAT LF IS THE ENEMY.
Arya was playing the game and winning, and you’re all bitching about her being just a murderous psychopath