The problem for the end of the Hobbit was always going to be the lack of a really compelling Big Bad: both Smaug and the Necromancer are seen off in the first third (which feels rushed to get to the battle) and the source books lack of a replacement (other than Bolg, who was killed by Beorn in the books rather than a dwarf, elf or hobbit) didnt offer any solution. Instead Jackson changed Middle Earth history, saving Azog The Defiler from death in the opening battle of the first movie and giving him elements of Bolgs story. Bolg then became a secondary villain, splitting one underwhelming villain in half, with somewhat inevitable results. The problem then was that two villains had to be dealt with, and the writing simply didnt match up to Tolkiens original. Instead of being skilled, genuine threats, both of them end up being killed because they both stand on some ice and then break it themselves. Seriously, it happens twice.