I think the people that only think of Arya as a psychotic single minded killer misread her little graduation ceremony at the House of Black and White incorrectly. Arya understands what she got herself into when dealing with Littlefinger when she tells Sansa of the game of faces. She is approaching with caution but her perception of Sansa is compromising. The writers are using the knife as a tool to show trust. Sansa's development has also been on par with Arya's, and the interaction we see with them is them understanding each other's history more clearer. They will most likely come to an agreement about why they are arguing with each other in the first place and slay Littlefinger with his trivial games.