Don't forget, it was the children of the forest that created the night king. They were in a war with the first men, who were kicking their butts, they weren't warlike, so they used powerful magick.
They captured a first man, probably a powerful warrior but we don't know, then they twisted him into something dark and powerful to destroy their enemies - the first men.
However, when you look at it this ls really a copy of Mary Shelly's "Frankenstein." They created a monster they could not control, and thus the night king went about to kill everyone. The first men because the magick forced him to, but also the children of the forest in hatred and revenge for what they did to him. The children of the forest have no idea how to destroy him or they would have done so long ago.
Remember too that Bran sees things as they need to come to pass. He told Samwell that "now is the time" to tell Jon who he really is. He didn't do it because of time, but because of timing. He had already seen it. He knew that Jon needed to be in just the right place and needed to be told at just the right time. So off he sends Samwell to do what needed doing. I think it is the same with the night king. He sees where the night king should die, and he sees everyone who will struggle. So he'll try to put all the pieces in place to get the job done. The only question is if the night king can see it too, how does he intend to cheat fate? Can he make a move that Bran cannot see, or that tries to take advantage of Bran's inexperience so that Bran isn't looking everywhere he needs to?
Doug
