From what we gather, he was one of the First Men who waged war against Children of the Forest. They captured him and transformed him into the first White Walker.
The idea of White Walkers was to protect the Children and the sacred trees but they turned on their creators and started the Long Night.
Why that happened, no one knows. The motivation of Night King never went beyond turning the world into an ice cube. As he is solely show’s creation, with the book’s Night’s King having no implication at being the leader of the Others, Night King remains a boring mystery.
Cheers.