That’s a pretty weak motivation IMHO.... Not saying its not all we are getting, but that’s almost a throw away... “They are doing it because they are evil”...that’s all...
Given this has been 80 episodes by the time it is all through, I would expect some more back story on the Night King and his motivations.
We will see if we get anything...
“That’s a pretty weak motivation IMHO (the Night King wants the Long Night to return)”
Maybe the three-eyed raven is lying, to manipulate people into fighting his enemy for him.
“I would expect some more back story on the Night King and his motivations.”
That seems to be the focus of the Spinoff prequel series that HBO already has in production - it is set 6,000 to 8,000 years earlier, tentatively called “the Long Night”.
So they have to maintain some mystery, purely for business reasons.