I think you’re mistaken on a few points.
One, it’s not true that very few humans died. Very few main characters died, but their army sustained major losses. Which brings me to my next point:
Two, the Night King is not the primary villain of the series. If he were, then I could understand your frustration. This would have been an extremely disappointing episode for me if it was the series finale. But it wasn’t. There are still more episodes and more battles to be fought.
I can understand why you might think the Night King is the main villain. Lots of characters, especially Jon, make the point of how all this petty squabbling over the Iron Throne is going to be pointless when the Night King and his army of the dead swoops down on Westeros.
But think about it. From a narrative perspective, who has gotten more character development? The Night King — who we still know almost nothing about — or Cersei?
Cersei is the main villain. And with their army significantly reduced, Jon and Dany are going to have a much tougher time claiming the Iron Throne. There’s still time for plenty of main characters to get killed off in the remaining three episodes.
“The Ice King dead so easily? Bah, never would have happend that way in the book.”
I’m not so sure that’s true. Even if it is, whose fault is that?
When the series began, Martin assured HBO and the showrunners that he’d have the series completed by the time they caught up to where he was. But he didn’t.
The last book was published eight years ago. Just a few months after season one aired, in fact. In the entire time this show has been on the air, Martin has not been able to finish a single book. And Winds of Winter still doesn’t even have a release date!