Arya is a highborn girl; an important person. And not just any highborn girl, but one of the most important half dozen houses in the realm. She ends up captured by enemies, but is dirty enough to be mistaken for a boy. Even cleaned, she isn't recognised. She's put to work cleaning on the cleaning staff at a castle, basically slave labor until the war is over (and after, the ones who really are lowborn won't have anywhere to go or anyway to leave, so they'll stay, and work, to keep getting fed).
After a while a hundred prisoner's from Arya's brother's army (her father died) come to the castle dungeons, and the enemy army leaves, except for maybe a hundred guards and some hired mercenaries. Arya manages to free the prisoners who take over the castle (the hired people all change sides). However, Arya doesn't trust anyone, so she doesn't tell them who she is, and keeps doing work. One day it is announced her brother's bannermen will soon leave, and she discovers she would remain and the hired mercenaries would rule the castle. They are *nasty* people. Really fucking nasty. Arya does *not* want to be in their power. So she decides to escape. She steals some horses and swords (two friends come with her) and food. The stealing seems perfectly moral to me. But anyway, after that, there's one thing standing between her and escape: the man at the gate. (She goes to a small gate with only one guard.) Arya kills him. One of her brother's soldiers, who did no wrong. Was this murder wrong?
I do have my own answer, but I won't give it until enough people comment. I do have one piece of advice though: I would suggest considering morality to be that which helps promote human flourishing, whether it's true or not in the limit, won't help at all here. Killing the guard is good for Arya's flourishing and bad for the guard's flourishing. Ho hum.