There is some evidence that suggests Bran may actually be the Night King.

Reddit user turm0il26 believes an episode to come could show a time-travelling Bran - now the all-seeing Three-Eyed Raven - warg his way into the man who became the White Walker's leader, in a bid to prevent the Children of the Forest from ever creating the army of the undead.

This theory revolves around footage from Season 6 where Bran wargs to see the creation of the White Walkers. Watch the video below: 

The way the 'Game of Thrones' producers filmed this scene indicates that Bran may have been in the man's body as he was turning into The Night King. The angle that Bran wakes up in is the exact same as the man tied to the tree. This may have been done entirely coincidentally, or it could be a subtle hint dropped by the 'Game of Thrones' producers.

Something else that further supports this theory is the fact that the Night King is the only one who can actually touch Bran while he is warging. The Night King's psychic capabilities may reveal something about Bran's involvement in the creation of White Walkers and the Night King.