the Ruler's motivation:selfishness; he wants everyone's approval, even the humans. At first, most people think that he's doing things to give the humans rights, and they are on board with it. But then they realize that he's been killing Trainers.
Pixar has become as famous for their storytelling chops, and their ability to create funny, inspiring tales that appeal to all audiences, as they are for digital animation. Here is some advice from the company.