Because it is a satire of the Soviet Union, Animal Farm uses its characters as representations of real-life figures. For example, Old Major represent Karl Marx, the originator of Communism. Orwell's position was that Marxist teachings always lead to dictatorships as ambitious people jockey for power, and this is best exemplified by Napoleon the pig. At first, he stays in the background, gathering power through subtle influence; his most important example of social organizing is the use of the sheep to shout slogans and drown out Snowball, making it seem like there is a united front against his ideas. However, Napoleon comes into his own when he appears with his private army of trained dogs, and takes total control of the farm.
Napoleon, with the dogs following him, now mounted on to the raised portion of the floor where Major had previously stood to deliver his speech. He announced that from now on the Sunday-morning Meetings would come to an end. They were unnecessary, he said, and wasted time. In future all questions relating to the working of the farm would be settled by a special committee of pigs, presided over by himself.
(Orwell, Animal Farm, george-orwell.org)
This is the first step towards marginalizing the other animals and raising pigs to a protected elite class, ruling and consuming without producing. In this manner, Napoleon mimics exactly the traits of humanity that Old Major decried, eventually walking on two legs and becoming all-but indistinguishable from humans. Napoleon's disregard for the central tenant of Animalism -- that all animals are equal -- echoes the inevitable beliefs of dictators, who make themselves and their friends into kings, while controlling workers with hunger, fear, and brutality.