What is Orwell saying about human nature in this book?
An addtion to the previous answer:
Orwell's animal characters also illustrate the famous quotation of Lord Acton: "Power tends to corrupt, and absolute power corrupts absolutely" (Lord Acton, in a letter to Bishop Mandell Creighton, 1887).
When the pigs first rebel against the farmer, it seems like a good idea. Humans really do take advantage of animals, so why shouldn't the animals fight back?
After the pigs take over, however, POWER begins to corrupt them. They begin to take advantage of the other animals, just like the farmer used to take advantage of them.
Orwell seems to be expressing a very pessimistic thought: that no matter who has power, that person or group will take advantage of those who are weaker. It's not the present leader is really evil by nature; it's that the power he has gained corrupts him and turns him to evil behavior.
It seems to me that Orwell is saying different things about human nature, depending on which character you're talking about. However, he's certainly not saying much good about human nature with any of the characters.
There seem to be two major kinds of characters in this book -- those who try to manipulate and dominate others and those who are so gullible and weak that they are easily manipulated. Examples are Napoleon on the one hand and Boxer on the other.
Looked at this way, Orwell seems to be saying that human nature is to dominate and control others if you are able and to be docile and gullible if you're not able to dominate. That doesn't seem very upbeat and hopeful to me...