I think things were going to the Night King's plan. He had his full force of White Walkers with him, having kept them away from the main battle and "sacrificing" only the wights. (remember if a White Walker is killed, all those he turned go down as well. The strategy of saving them to act as a phalanx for the Night King was not that bad of an idea). The Dothraki have been pretty much destroyed. The Unsullied have suffered major losses. The wights have over-run the walls of Winterfell. The living dragons are riderless (and the dragon-fire failed to engulf him). As a deterrent to Jon Snow, thousands and thousands of additional dead have been raised to keep him occupied (as well as having Viserion as a distraction). The dead are rising in the crypt. Theon is the last man standing in the weirwood. Now is the time for the Night King to come and claim his final victory. To do it himself--to give the coup de grace to the Three-Eyed-Raven. All things are in place for him. Except. No One is there. I absolutely loved this. Arya with her years of training (ALL the training: Winterfell; Kings Landing; on her escape from Kings Landing; Harrenhal; and finally Braavos) brought her to this moment. That she slipped past the White Walkers with only a breath of wind stirring the hair of that one WW. He looks briefly, but sees: No One.
Yes. His plan was going nicely. His arrogance as well as No One brought him to his end.
....or did it???? Either way, for me this was perfect!!!!!