One of the major themes of Animal Farm is the use and abuse of power. The idea of this story is to show how power currupts those who possess it. The more power the individual has, the further they stray from just idealism.
Napoleon and the other pigs are examples of this corruption. The pigs start off trying to build a Utopia for all animals, and because they were smarter than most of them, they became the leaders. However, once in power, they started taking more and more for themselves because they felt entitled. Even Snowball and Napoleon agreed to take the milk and apples for themselves and not divide them among all the farm animals.