Now that everyone is more calm, here is the reality
1. The entire scene where Arya was sneaking in the library + her entire storyline was made to make us aware she is silent and sneaky, thus she was able to sneak on the night king
2. Classic 'twist' where we get to know (as we could've speculated all along once Cersei didn't join the battle) that the real enemy was her and not the Night King, he was just made to seem so as a plot line to distract us from the real darkness (it's not that uncommon)
3. The game is titled Game of Thrones after all, the throne being the IRON THRONE, the real battle is for that
4. How else would they kill off Cersei sitting in the South and let the 'good guys win' (if they do), without rushing the final plot of the series, if they hadn't ended the night king bit early?
5. Realistically, the Humans had no chance of holding off the undead for longer than they did, so they had to either win or lose fast, we all know GoT is fast paced
peace homies calm down and enjoy the show