The way the fight should end between night king/ walkers and Snow's army. During the battle, Jon Snow and Night King are fighting and the NK is getting the best of Snow and has him down and about to kill Jon Snow. All of a sudden the NK is stab from behind and shatters along with all the other walkers. Jon Snow looks up to see 1 Walker standing there. The Walker's hand goes up to his chin and pulls off his face and it's Arya and she reaches down to help Jon up.
She would have to have one of their faces.. I suspect she may be the one to kill him but we have been a couple of episodes now with no major character dying.. so I expect a lot of that soon.