I'm guessing that the people that complained about the ending didn't know that the movie had been split up into three movies. Though you'd think that people going to see an advanced screening would be in the know enough to realized this fact. And if they did know, how can they complain about a fact that they were already aware of before hand.
I believe I heard Peter Jackson was pulling in additional content from other sources. I'm not so versed in the lore I know everything, but it's likely that the included character wasn't something Jackson made up, but something that appeared in another book.
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