The humans in the story are separate from the animals on the farm. In a way, this separation and difference is the human's most important characteristics. This distance in culture (ways of dressing, ways of walking, etc.) also takes the form of an emotional distance. When the humans do not feel a connection with the animals, they are willing to treat the animals very poorly and as lesser creatures without rights.
Over the course of the story, the pigs lose their connection with the other animals on the farm. Though they begin as comrades and equals with the other animals, they end up being "more equal" than the other animals on the farm, superior and emotionally distanced.
...in the novel's last scene, Pilkington chokes with amusement as he says to the pigs, "If you have your lower animals to contend with, we have our lower classes."
When the pigs begin to feel that they are no connected with the other animals on the farm and can treat them as if they have no rights, the pigs have become like the humans.
Why do the pigs become disconnected from the other animals on the farm? This is one of the central comments of the novel. The answer relates to the dangers of political power. Power corrupts.
Orwell knew that with power came the abuse of power and only a vigilant citizenry could prevent such abuses.
The pigs separate themselves from "the citizenry" and become elevated figures. Literally drunk and also drunk on power, the pigs participate in forming a hierarchy that serves them and that offers nothing but disservice to others.
The pigs become like the humans by the end of the story as throughout the story they have revealed similar traits; they want to seize power and maintain control over all the other animals.This is exactly what the humans always did, and the pigs, in effect, take over from the humans and rule in just the same oppressive manner. In this way they wholly destroy the original ideals of the revolution which called for complete equality for all animals and the rejection of all human contact. By the end of the story the pigs are in open alliance with the humans and behave just like them, even walking on two legs like them. This is why they end up appearing quite indistinguishable from the humans.