To me, the major theme is that power corrupts people. I think that Orwell is trying to tell us that even people who start out very idealistic and become corrupted. The main idea of the story is that retelling the story of the emergence and development of Soviet communism in the form of an animal fable.
Animal Farm is richly enigmatic. Animal Farm belongs to a quite different tradition, in a line of moral animal fables which goes as far back as Aesop. One could call it the animal fable based on socio political ideas, aimed at an adult as much as at a child reader. Animal Farm, began as a tract with a political motive. Farmer Jones’s Manor Farm is an Orwellian Lilliput, satirising the pretensions of the Russian Revolution of 1917. Its prompt corruption by a new, more ruthless power elite.