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From Sparknotes.com:

"Animal Farm is most famous in the West as a stinging critique of the history and rhetoric of the Russian Revolution. Retelling the story of the emergence and development of Soviet communism in the form of an animal fable, Animal Farm allegorizes the rise to power of the dictator Joseph Stalin. In the novella, the overthrow of the human oppressor Mr. Jones by a democratic coalition of animals quickly gives way to the consolidation of power among the pigs. Much like the Soviet intelligentsia, the pigs establish themselves as the ruling class in the new society.

The struggle for preeminence between Leon Trotsky and Stalin emerges in the rivalry between the pigs Snowball and Napoleon. In both the historical and fictional cases, the idealistic but politically less powerful figure (Trotsky and Snowball) is expelled from the revolutionary state by the malicious and violent usurper of power (Stalin and Napoleon). The purges and show trials with which Stalin eliminated his enemies and solidified his political base find expression in Animal Farm as the false confessions and executions of animals whom Napoleon distrusts following the collapse of the windmill. Stalin's tyrannical rule and eventual abandonment of the founding principles of the Russian Revolution are represented by the pigs' turn to violent government and the adoption of human traits and behaviors, the trappings of their original oppressors.

Although Orwell believed strongly in socialist ideals, he felt that the Soviet Union realized these ideals in a terribly perverse form. His novella creates its most powerful ironies in the moments in which Orwell depicts the corruption of Animalist ideals by those in power. For Animal Farm serves not so much to condemn tyranny or despotism as to indict the horrifying hypocrisy of tyrannies that base themselves on, and owe their initial power to, ideologies of liberation and equality. The gradual disintegration and perversion of the Seven Commandments illustrates this hypocrisy with vivid force, as do Squealer's elaborate philosophical justifications for the pigs' blatantly unprincipled actions. Thus, the novella critiques the violence of the Stalinist regime against the human beings it ruled, and also points to Soviet communism's violence against human logic, language, and ideals."

The pigs with the exception of snowball were filled with power and blinded by absoulte power therefore they became corrupt because they want more and more with out any rewards to the other animals.