How do Napoleon and the pigs betray the principles of Animalism?
Napoleon and the pigs betray the principles of animalism first by reserving privileges for themselves that are not allowed to the other animals after the revolution. First they consume the milk while the other animals are bringing in the harvest. It is revealed later that the milk is mixed into the pigs' mash because, they say, preserving their health and well-being is essential for the success of Animal Farm. They are also eating apples with their mash, unlike the other animals. In justifying their act Squealer appeals to the fears of the other animals, claiming that the farm is always in jeopardy of being overrun by humans.
As the plot moves on, Napoleon arrogates more power to himself, chasing off Squealer using the dogs he has raised, and ending the animals' Sunday morning meetings. He declares that he will run the farm himself, basically as a dictator. Napoleon also begins to cultivate relations with humans outside the farm, selling wheat to people even though there is barely enough food for the animals to eat. The pigs move into the farmhouse, where it is revealed they are sleeping in beds. He purges the chickens and several of the young porkers who are protesting his decision on the pretext that they are in league with Snowball. Indeed, Snowball is portrayed by the pigs as in league with humans, always trying to overthrow the farm's leaders and return the animals to slavery:
The animals were thoroughly frightened. It seemed to them as though Snowball were some kind of invisible influence, pervading the air around them and menacing them with all kinds of dangers.
The pigs use this fear to justify their abuses, which reach another level when Napoleon and the pigs begin drinking alcohol. As is their practice throughout, they change the commandment against alcohol to conform with their new practice.
By the end of the book, the animals see Napoleon and the pigs in the house, playing cards with the humans, and the pigs have changed so much that they cannot distinguish between them and the people.
The pigs initially begin to reserve certain privileges for themselves by adding milk and apples to their mash. They justify their need for milk and apples by saying that they are brain-workers and must eat the nutritional food in order to prevent Jones from returning. Napoleon then eventually usurps power by chasing Snowball off of the farm and begins to rule Animal Farm like a tyrant. He begins to betray the tenets of Animalism by making business deals with humans. Napoleon and the other pigs also occupy the farmhouse and begin sleeping in Mr. Jones's bed. Squealer manipulates the tenet by adding "with sheets" to the Fourth Commandment. Napoleon continues his tyrannical reign by slaughtering the animals who disagree with his leadership opinions and reducing the animals' food rations while simultaneously making them work harder. Eventually, Napoleon and his pigs begin dressing like humans and drinking alcohol, which is also against the tenets of Animalism. They walk upright like humans on two legs and change the title of the farm back to its original name. By the end of the novella, the other animals cannot tell the difference between the pigs and the humans arguing in the farmhouse.