Examine the corruption of the pigs.
The corruption of the pigs represents how power can be a quality that takes away from an individual's moral or ethical center. The pigs become the forces of Animalism. While Napoleon had always demonstrated a penchant for power, it becomes their ascension into the realm of power where their corruption becomes most evident. Power is the force that enables their corruption. Having the ability to live in Jones's home, being able to use their ability to read as a wedge to deny other animals a share of power, and being able to justify their excesses and indulgences as part of their "responsibility" to the other animals become the means by which their corruption continues. As the farm becomes more centralized in the power of the hands of the pigs, their corruption increases. Napoleon is able to use Squealer to "justify" the corruption and the dogs to substantiate his own claims to power.
The corruption that Orwell displays is one in which political power becomes the tool through which civic and personal corrosion increases. There is an absolute tone in which Orwell speaks of how personal corruption results when there is an ascension of political control. The corruption of the pigs reflects this consolidation of power. The ending is one in which there is no moral or physical difference between the pigs and humans because both have become corrupted through political power.