The entire book really builds up to this giant climatic confrontation.
The author pretty much chickened out when she didn't have a big fight at the end of BD. She admitted that she didn't want to kill any of Bella's new family. Even many fans that read the books were disappointed.
But the screenplay writer has made changes when she thought it would make a better movie. They already put in the sex scene in Part 1 when it was skipped in the book. A big battle will be put into the Breaking Dawn Part 2 movie. Having the last movie end without a big fight would be terrible and hurt how much money the movie makes.