In Animal Farm, Napoleon (with Squealer and Snowball) developed Old Major's ideas into the principles of Animalism. So, initially, it appears that he supports the true spirit of the Rebellion, which was to make a society in which all animals are equal. However, he soon orchestrates his way to becoming the sole leader and while doing so, he separates himself from the animals who he was supposed to have been equal with. In the end, Napoleon is an easily corrupted opportunist with no consideration for the other animals.
Napoleon is based somewhat on his historical counterpart of the same name; in other words, a deluded commander who attempts to take over the world. But Napoleon is more closely based on Stalin, who made a secret deal with Hitler (Mr. Frederick) while he was allies with England. Like Napoleon drove the Animal Rebellion into the ground, Stalin drove the Russian Revolution (which at the beginning, also promised a more equal, communal society) into an oppressive dictatorship where many people suffered and died.