At the beginning of the book, Old Major casts a vision of a new type of society. From the thralldom of human rule to freedom among animals. However, as the story progresses, Animal Farm becomes just like human society. Let me give you a few examples.
First, there is hierarchy. The pigs are the ones clearly in control. After a while, it is Napoleon who is in control. Moreover, if anyone disagrees with him, Napoleon will use "muscle," the dogs, to get his way, even if it means killing the other animals. There was a veritable slaughter. Here is the text:
They were all slain on the spot. And so the tale of confessions and executions went on, until there was a pile of corpses lying before Napoleon’s feet and the air was heavy with the smell of blood, which had been unknown there since the expulsion of Jones.
Second, there are laws. There are seven commandments. At first, these commandments seems equitable, but later they are changed by the pigs to serve their needs. Hence, we can say that there are not only laws, but also unjust laws that serve those in power.
In light of these points, at the end of the book, it is no wonder that Napoleon turns into a man. He is Jones, the worst parts of Jones, and along this fact, he represents society, the worst of it, too.