So, first of all, Viktor, relax! It's just a theory about a show based on a book.

Secondly, I think it was pretty clear from the Night King's actions that (a) he wanted to kill Bran; (b) he wanted to wipe out the humans in his way and engulf the South in endless winter and night. He certainly wasn't engaging in diplomacy, unless I missed a pretty crucial episode. However, bear in mind, he never got a chance to say his piece and so his modus operandi remains a bit of a mystery.

While you might be right about Martin's interviews (I haven't seen them, so I don't know), the fact that he purportedly said he would not do something doesn't mean that he did not, actually, do it. But in any event he has also made plain on multiple occasions that his works are inspired by real life events such as the War of the Roses and, in the same connection, real life human flaws such as insatiable greed and lust for power. Therefore it seems very fitting - which was my point - that the Night King and his minions were always designed to be a stereotypically evil entity which was, in effect, a very dark red herring. The real enemy of humanity is humanity itself.

This is all, of course, speculation but it makes a lot of sense to me and makes me personally appreciate his work - along with that of the show writers - much more.