Sometimes the mentor who was established in the beginning of the film/TV show was not who we thought he/she was.
Thorin Oakenshield in the Hobbit trilogy was introduced to us as a noble, just and determined leader set on reclaiming his lost homeland, and for a long time that is precisely what we were given. However, come the third installment of the franchise, we see that the greed of the dwarves of Middle-Earth has risen to the surface of his character as Smaug, the last great firedrake of Arda was slain. He lusts after the great treasure hoard of Erebor just as Smaug had (dragon-sickness). He was even willing to kill Bilbo when he heard that he had given away one piece of treasure (then again, it was the Arkenstone, the crowing jewel of the treasury of Thror). Thorin turned into a complete monster, though thankfully he realised the error of his ways by the end of The Battle of the Five Armies.