Endings can be hard and there will be all sorts of constraints on a production like that as so much money is involved. Adapting books is always a compromise. It has to appeal to people beyond the dedicated readers.
You can look at Shakespeare for stories of war and ambition. Those were grim times that make today look civilised, but we still have some similar conflicts driven by people who want power.
Yeah, but they ran out of books to adapt for GoT; the last two books still aren't out. Apparently George is having difficulty finishing it, too.
The story goes that he told them the major plot points, like who ends up on the iron throne, but they made up all the filler on how it gets there.
Directors can't resist messing with stories anyway as they need to tell them in different ways. Peter Jackson did it with the Tolkien movies too.