Bran has been through a lot since the start of Game of Thrones. I remember exactly how I felt watching him in the ever first episode. "Oh look at that little scamp! You best hope your mother doesn't catch you! Wait a minute....Jaime and Cersei, what are you doing!? OH NO BRAN WATCH OUT!"
After being crippled by The Kingslayer, you would be forgiven for thinking that's pretty much Bran out of the picture. But then he kept having those strange visions of a Raven with three eyes. Sure at the time we had no idea what that was about, but we were determined to find out. Since fleeing Winterfell and crossing The Wall, Bran has met the mysterious Three-eyed Raven, learning of him the Art of Greensight.
With his new found talents, Bran is a most welcomed friend against the evils of the White Walkers. Or, is he? As painful as it is to say, but Bran has far more in common with The Night King now then he does with even his own family. In fact, there's a lot of evidence to suggest that Bran and The Night King might be gearing towards becoming allies in the future.
Bran was trained with the very people who created The Night King. During his training, the Three-Eyed Raven shows Bran a vision where Leaf and a group of Children of the Forest insert Dragonglass into a captive human's heart, transforming him into the dreaded Night King. When Bran is reasonably shocked that his teachers are responsible for the biggest threat mankind has ever faced, he questions why Leaf would create such a monster. She replies "We needed to defend ourselves. From you. From Men." If the Children of the Forests deemed humans so dangerous that they felt they had to create an entire race to destroy them, would they then teach their magic to someone they consider an enemy? You can clearly hear the fear in Leaf's voice when she tells Bran about her war with The First Men, so she still must consider humans a threat. Maybe the only reason she wants to destroy The Night King is to replace him with someone she can control.
That ship might have already sailed for Leaf though, as The Night King himself seems to want Bran for himself. When Bran manipulates the Weirwood Roots to grant him a vision of The Land of Always Winter, The Night King marks him. This allows the Night King to always know where Bran is and neutralises any magical wards protecting him. He proves this by attacking the Cave Bran has been training in and killing the Three-eyed Raven. Now, this attack could simply be The White Walkers killing anything that moves, or it could be The Night King playing the long game. If I was a magically zombie who wanted to kill any living human I came across, I would realise that the best way to learn what's happening on the other side of the wall is to have a spy tell me what those tasty humans are up too. By forcing Bran to flee back to civilisation, he has become The Night King's eyes and ears, regardless if Bran wants to be or not. The Night King has shown that he knows when Bran is using his powers, so he can use that to his advantage by showing the boy only what he wants him to see. If you think I am giving the Walking Ice Corpse too much credit, remember that he has already laid one trap for Dany's Dragon, Viserion, He knew Dany wouldn't let Jon Snow and the others die and he also knew that she would use her Dragons to save them. Cos it's Dany, she uses her Dragons for everything. He got the real prize of a Dragon to raise, giving him a way through the wall. If The Night King is clever enough to do that, he's clever enough to manipulate Bran.
That's of course if The Night King would even need to manipulate Bran. With his new-found powers and his disregard for the position of Lord of Winterfell, Bran is showing us that he really can no longer relate to the humans around him. Remember how seriously Bran took the role of Lord of Winterfell when Robb Stark was leading his rebellion against the Lannisters? If present day Bran was in that position, I think he would have stopped caring by afternoon tea. Bran has an understanding of magic and nature no other human can possibly relate too. For all intend and purposes, Bran IS no longer human. What if he decided that The Night King is right to wipe all of Mankind away? What if he decides that a land ruled by the dead is better than a land ruled by the living?
I am the last person that wants Bran to turn his back on humanity, but you can't deny it's a possibility. After all, the world before Bran gained his Greensight powers crippled him, sent assassins after him and ran him out of his home. Why not burn it to the ground?