I think that alot of people are upset that the NK is dead after 1 real fight episode against him. Its been buildt up ever since the very first episode, even the very first scene, that the dead will come. It may have served as a "distraction plot" from the very beginning, but it quickly became something much more for alot of fans.
Combine that with the fact we didnt really learn more about the NK and his army, the arya move felt a bit to thin. I dont think the kill in itself is the biggest issue here, but that the fans feel that something is missing. what was the purpose of them? The NK cleary is more than a braindead zombie, you could see that from his smile after he got burned. Here I think alot would be resolved if we got something more out of Bran, but as usual we got nothing. He obviously knew what was going to happen though.