I think the WWs have some hidden agenda, I don't think the Night King is just there to fucking MURDER everything, he's got something else going on inside that brain of his. He, and his WW friends, are clearly very, very intelligent. They display emotions such as anger, shock and some form of happiness, and it'll be a big reveal when we finally find out what the fuck they want.
Some people speculate that's it a Lich King situation: That the WWs are all controlled by the Night King, who was created to control them and keep them from Westeros, but now the NK is pissed and wants to fuck everything up, and some people think the ending will involve someone sacrificing themselves to become a new NK after the old one is murdered to keep the WWs up North.