People love to theorize about Game of Thrones and some of these theories are made by people wearing "tin foil hats," meaning that a theory is way, way out there. Like the theory that Bran is the series' true villain. Here is why Bran can't be the Night King on Game of Thrones, as delightfully weird and twisted as that would be.
The theory, first proposed by Redditor turm0il26, supposes that Bran is both getting better at his time-traveling warging abilities, and still does not quite understand them. I'm with this so far. But here is where it gets confusing. This theory posits that Bran tries to stop the White Walkers from ever happening by traveling back in time and warging into the human that was to become the Night King. He would try to stop the Children of the Forest from stabbing him with the dragonglass, but they wouldn't understand him, and in this way he'd get stuck as the Night King, which is how he now exists simultaneously. On the surface, it could be possible, but there is basically no evidence to back it up. And on a purely narrative level does not work because that is not how character development works.
The best theories and the best twists always feel earned in some way, and Game of Thrones is very good at doing that. The Red Wedding, for example, was horrifying and shocking and yet afterwards felt totally inevitable. Of course Robb was going to be young and naive and marry a beautiful girl he loved. Of course the Freys were going to be furious that he broke the engagement to one of theirs and ruined a powerful alliance for them. Of course Cersei and the Lannisters were going to try to capitalize on that in the most brutal way possible. Another good example is with Hodor. Viewers never knew why Hodor only said Hodor, but when it became clear that he died holding the door for Bran while Bran was warged into his past self, the result was a beautiful and heartbreaking moment that felt logical.
From a narrative perspective, it was clear that Bran didn't believe the Three-Eyed-Raven when he said that the past cannot be changed. In the story, the purpose of Hodor's death was to prove that very point to Bran. Hodor was always going to be Hodor because of what Bran did in the past and there is no way to change it. This is a lesson Bran in fact did learn the hard way, and to have him go back in time to try the same shenanigans again would just be bad story telling.
Then there's the fact that for this to work, Game of Thrones would have to expand Bran's time-travel powers rapidly, and spend a lot of time talking about the ins and outs and mechanics of time-travel to the detriment of the rest of the story.
So no, I don't believe that Bran is the Night King. But I still enjoy the theories.