There's a wild theory online that Bran is actually the Night King. The Bran figures out a way to not just view the past, but become a physical presence in the past. Which is why he is also all of the Brandon Starks in the past. They talk about him know every winterfell stone better than anyone, this is because he is Bran the Builder and actually built winterfell. And then he goes back and tries to warn the children of the forest, but because he is a human, and they're at war with the humans, they capture him, and turn him into the Night King. That scene where he watched how the Night King was created, was watching himself. Apparently he winces when watching the dragon glass stabbed into the guy (him?), and his physical form mirrors that of the human(him?) tied up in the vision.
Which also fits with GRRM's bittersweet ending. Having Jon or Arya, having to kill their brother to save the realm. etc etc
Or so that's the theory.