Why do animals behave like humans, and humans like animals in Orwell's Animal Farm?

I believe that Orwell's point was to demonstrate that human beings are capable of displaying characteristics of animals under certain circumstances, and that animals are more intelligent than humans credit them. The desire for absolute power and control exists among both species. 

The farmer was obviously an "ignorant" character, one lacking education or even social "graces" according to social mores. Ignorance in humans can often be a catalyst for animalistic behavior, such as murder, abuse, and desire for survival at any cost.  What a person doesn't understand, he is often afraid of, as in the farmer's case. The farmer was determined to survive, at any cost.

Pigs are considered to be the most intelligent of all animals, with the exception of chimpanzees.  Pigs are an appropriate choice, given the setting of the story, as well as the characteristic of higher intelligence.  Orwell chose to give the animals the gift of speech in this novel, which enhances both plot and characterization.  The pigs are the dominant characters because of their intelligence, while the remaining animals follow their lead, almost in blind submission.  This results in the pigs developing a "pecking order" among the animals, as often seen in human society.  The pigs begin to assume more human characteristics, one being the need for justice as they feel oppressed by the farmer.  In history, oppression often leads to a "coup" to establish justice and stability for those being oppressed by those in power.

The pigs develop a brilliant plan to take control of their farm, but in the process, there is dissent among the pigs, which filters down to the "lower" animals.  In human nature, it is difficult for more than one person to wield absolute power and control, as in a dictatorship.  The pigs begin to argue about details of their plan, with an underlying current of who is and should be in total control of the animals when the farmer has been supplanted by the pigs. 

At the end of the novel, we see the culmination of the struggle for dominance and power among the animals; this dominance by the pigs will be attained at any cost, a human trait that has been demonstrated repeatedly throughout history.