i saw it as doing GRRM a favor, if they kill off hound on screen it insinutes itself that hound is definitely dead in the books. they have already killed off several book characters on the show that are still alive in the books, but none of them are as important as sandor. so yeah they left it open ended and yeah i think they made arya look like a cold blooded cunt on purpose (due to her decision to become a faceless men) even though it doesnt jibe too well with their budding friendship to that point. remember, he was still on arya's list up until then, so she probably thought by not killing him it meant she was leaving his destiny up to the gods even though he deserved to die.