It would make no sense to provide some big motivational reveal for the white walkers. We know everything we need to. They are former men, they were corrupted by the children of the forest by shoving a magical obsidian blade into a man's heart in an effort to protect against the invasion of men who were bringing death and destruction, as men often do.
White Walkers are simply the personification of the evil that men do. Their association with cold and death and winter are one of the few things about GoT that Martin intentionally designed to be stereotypical fantasy. They are basically a MacGuffin.
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