Concerned?I'm always slightly concerned about people who make moral judgements on characters from an entirely different (and fictional) universe based on contemporary moral social standards.
FICTION!

There is no one correct way to enjoy fiction and there's no rule that says you have to check your morality when reading/viewing it. I also tend to adopt the ethics of the heroes/anti-heroes in a story as I feel it aides in immersion, but I don't chide people for not doing it. And we all have our limits (all of us that are not sociopaths, that is), as I'd find it difficult to find no fault in a character who espouses positive views about things like rape and slavery.
Doesn't lessen my enjoyment of the material, if it's good and the reprehensible actions are in context. But it being fictional or contextual doesn't make the act itself any less abhorrent. And remember, Irvy, most times the writer wants you to be affected, to challenge your preconceptions. Arguably, ASOIAF or something like Macbeth is built on this as a premise. That the way one is affected would be so variable is not a fault (of either the writer or the audience) but is instead exactly the point.
I don't find Arya's actions all that appalling, given the circumstances and the character's evolution, but I can understand why others would.