One of the startling bits in Arya's story is when she meets Edric Dayne. Ned is older than Arya and is a squire to Dondarrion. He lives a dangerous life but then we have this interaction:
Edric Dayne: I won a prize once, riding at rings.
Arya Stark: I never learned the lance, but I could beat you with a sword. Have you killed anyone?
Edric Dayne: I'm only twelve
This is not Arya being a murderous child who is too far gone. Death has become a natural part of her life at this point, and it is not some philosophical truth but a truth of the violence of war.
" I killed a boy when I was eight, Arya almost said, but she thought she'd better not. "You've been in battles, though.".
isn't her trying to subtly measure her kill counts to those of 'seasoned men'. This is her desperately trying to reach out and understand if this heaviness she feels inside is something peculiar only to her ( and she does, we know she does when she has nightmares about killing men like Polliver).
The "she thought she'd better not" carries a similar emotion as her thinking that Catelyn wouldn't want her if she comes to know about the stableboy. In fact when Ned Dayne reluctantly agrees that yes, he has been in a war ( Arya picks up how he isn't exactly proud of it) but says he only stood guard over his injured lord, Arya remembers the stableboy at KL and the guard at Harrenhal.
She didn't know if Weese and Chiswyck counted, or the ones who'd died on account of the weasel soup . . . all of a sudden, she felt very sad. "My father was called Ned too," she said.
we have Arya's "oh..." moment. We have her going " Oh, so it has not been the same for everyone. Oh, they don't have their hands as bloody as mine. Oh, I have lost my childhood to war. " Her sudden shift where she talks about Eddard Stark is then a heartbreaking remembrance of a time when she too had her own rose glasses on, when knights were heroes and brave, when her father sat with his men amidst the walls of Winterfell and she was only Arya Underfoot.
You know I can't help but compare this to show!Arya where she flippantly tells Lem that she has shot the strawman perfectly in the head, tits, crotch. I couldn't figure out where D&D lost sight of Arya Stark and I think now I may have an idea. When left to their own D&D thought that in a world gone to war, Arya would thrive on violence. But she has only ever thrived on kindness.
Because it is kindness that makes Lady Smallwood such a noteworthy figure in her storyline, it is the kindness of her Braavosi friends that makes her love the alias of Cat ( can't believe that in the showverse! even this got tainted by violence with the stupid af waif storyline). Why she thinks Hot Pie would like the kitchen of the HoBaW: he stole tarts to give to her.