I think it's both. On the surface (and what she shows to him in her behavior and words), Arya is leaving The Hound to suffer and bleed prefusely, and die on her terms - devoid of mercy, similiar to how he treated his victims. A part of her despises him, his previous malicemalicious actions and what he stands for, he. He killed her friend, served the Lannisters, failfailed to save Catelyn and Robb at the Red Wedding, he had her captive and up for sale - and he is an early name on her "To Kill List"; onList".
On the other hand, deep inside she has developed feelings of friendship and gratitude towards him as well, knowing he saved Sansa from rape, he despises Joffrey and the Lannisters, he basically saved her from her mother and brother's fate at the Red Wedding, he was her companion and some kind of a guide to her (teaching her mostly violence, naturally for Game of Thrones), and in a way she now owsowes him a debt which is problematic because he's on her list.
In my opinion, it's not that she couldn't do it morally... It's that she didn't want to. And she was convinced he was dying in agonizing pain anyway. Watch Season 6 knowing all this... 😉
