Another theme of Animal Farm is that power corrupts those who possess it. How does Orwell bring out this idea through the character of Napoleon?

The animals rebel and take over the farm. The original intent of this rebellion is to make the lives of the animals better. The goal is for the animals to control their modes and means of production. They will determine their own work schedules and they will reap all of the benefits of their work. This is basic communism and if practically and responsibly applied in this small community, the animals could/should be better off. In the beginning, their lives are better. However, like other historical examples of communism put into practice, their live gradually worsen and the leaders of the acquire too much power. 

Napoleon is a representation of Joseph Stalin, a leader of the Russian Revolution and eventual leader of Russia. He, like Napoleon, increasingly gained more power and the people of Russia (like the animals of the farm) saw living conditions worsen. 

Napoleon becomes more and more like a human. He distances himself from the other animals. This is contrary to Animalism (Communism). The original narrative of the rebellion is to show solidarity and unity among all animals, Napoleon included. But in his increasing desire for power, he clearly demonstrates that he (and some other pigs) require better nourishment and living conditions than the other animals. He raises and instructs nine puppies which will become his secret police, further isolating himself from the other animals. He ousts Snowball from the farm so he can have more power. At the end of Chapter VII, Napoleon has animals executed under the suspicion that they disagree with his policies or have rebellious thoughts. Napoleon also has Squealer occasionally change the tenets of Animalism to suit his growing greed. Each change signifies his increasing power and shows how the Animal Rebellion has gone further and further from its original intentions.