Here is what I read.
Game of Thrones fans have given us some pretty wild theories this season, but this one takes the cake, so please prepare to have your mind blown. According to a casual genius lurking on Reddit, the Night King is immortal and there's no way for Jon Snow to kill him. In fact, the only way to get rid of him is to casually become him. .
"Someone has to 'pull out' the dragonglass in his chest which was used to create him in the first place. By doing this the Night King would 'die,' but unlike what happens when someone kills a 'normal' white walker, none of the creatures the Night king 'created' or turned would die. Instead they would lose the mind controlling effect the Night King has on them, and they would start doing whatever the f*ck they want, i.e. rampaging around the country. To prevent this scenario from happening someone has to take the place of the Night King by stabbing himself with the previously extracted dragonglass-dagger. By doing this the person (in our mind Jon Snow) would sacrifice himself for the greater good, gaining control over all of the wights and walkers. He then would lead them back over the wall back into the north where they would stay like it has been before since the last long night (like a beast in a cage)."
Literally, us/Jon Snow right now:
This theory, known as the Davy Jones Theory because of its similarity to Pirates of the Caribbean, is also linked to the Azor Ahai lore. Reddit claims that Azor is the original Night King, took up the job in a moment of sacrifice (like Jon Snow will), and even helped build the wall. Crazy, right? Meh, not really:
"After that the Night King secluded himself far in the North where no one else could survive. Now over time (in the many years since the Long Night) the personality of the former Azor Ahai slowly began to decay and he finally turned into what we see now in the series as the Night King. At some point the vanishing personality of Azor Ahai could no longer hold in check the 'evil' that was lingering inside him. At that point the Night King started moving again, returning from the far North and slowly started creating new white walkers / wights as he had before. And thus creating an never ending, ever repeating cycle of someone becoming the Night King (saving all living beings), secluding himself in the north, losing his personality and returning as evil."
