Arya is already evil.
She murdered a stableboy in KL. She ordered the deaths of Weese and Chiswyck. She conspired to murder the Brave Companions. She murdered one of Bolton's guards. She killed the squire when Polliver and the Tickler attacked Sandor. She stabbed the Tickler in the back and repeatedly stabbed him until he was dead. She murdered Dareon.
I'm not saying that those men did not deserve to die. And in the cases of the stableboy, the squire and the Tickler, it could be argued that her life was in imminent danger... and that she was defending herself.
But what had Chiswyck and Dareon done to her? They were liars and cheats without any honor, but they'd done nothing to her. Arya is not an appointed judge nor a licensed executioner. And she did not kill or order people killed out of a sense of justice (well, maybe in Chiswyck's case). She murders or has people murdered to satisfy her lust for vengeance. You don't agree? What is the last thing Arya does before sleeping? Ah, yes, she recites a list of names of people who she wants dead.
And what is Arya doing now? Oh, yeah... she is in training to become an assassin. She does this to further her skills so that she can personally kill everyone on her list.
Arya is evil.
Boaz - Robin and Vizion (hey guys) pretty much summed it up for me, but I just can't leave it alone. Here's how I'd break it down.
Stable boy = him or her.
Weese = A vindictive, childish, choice, but a vindictive man.
Chiswyck = more than justified.
Brave Companions = more than deserve to die.
Bolton's guard = incredibly harsh, but necessary. I'd say this was actually the most unpleasant of the kills though.
Polliver's squire = pretty much, again, him or her (he may not have killed her but what exactly do you think Polliver and the Tickler would've done to her?)
The Tickler = cheered every stab.
Dareon = again, harsh, but screw him - he abandoned the Night's Watch. In itself a death penalty.
How anyone can say that she has no right as judge and executioner is beyond me. We've seen time and again that people with the 'lawful' rights of judgement and sentencing are constantly abusing their power (the only exception, tragically, was Ned), while everyone else - like the brave companions - simply couldn't care less.
As we know, this is not a world full of modern sensibility towards life and death. Arya is simply responding to the disproportionate amount of suffering and abuse that she has endured and seen everywhere around her. Her 'lust for vengeance' was born and conditioned by those events and, in my opinion, is completely understandable. How else is a child who's lost everything supposed to respond? Hell, how would the Jaime of old, or the Hound, or Gregor respond to having everything they valued utterly destroyed?
If Arya is evil, then there isn't a word for Gregor and his men. There's no question that she's on a slippery slope in terms of where Martin's going with the assassin angle, but until she slits Jon's throat I'd like to think that the preservation of Needle means she still hangs very much in the balance.
And besides, you said she was evil - you didn't say if you
hate her.