I think that Orwell does not have just one view of human nature. I think that he believes that people in different positions will have different natures.
For example, he seems to think that power corrupts people. He thinks that whoever gets power will end up acting badly. He illustrates this with the way that Napoleon takes all the power for himself and then starts to change until he eventually becomes just like Jones and the people.
But Orwell thinks that "lower" people have different natures. They are basically selfish like Mollie, or stupid like Boxer and the sheep. This is a very different nature than the grasping, ambitious nature shown by Napoleon.