In Animal Farm, Napoleon might've had aspirations of power prior to the Rebellion. However, the implication in the novel is that power can lead to corruption and absolute power can corrupt absolutely. When Napoleon begins to isolate himself from the other animals, he creates a class system in which certain groups of the animals are treated differently. With this structure in place, he and the other pigs further isolate themselves from the other animals to the point that they (Napoleon and the pigs) become just like Mr. Jones in terms of how he/they mistreat the animal workers.
Snowball recognizes the windmill for its power and its ability to make lives easier for the animals. Napoleon recognizes Snowball as a threat to his own power and subsequently chases him off the farm. Napoleon then claims he championed the idea of the windmill all along. However, he looks at it for its productive powers; he doesn't consider how the windmill can help improve lives of all the animals. With Napoleon, it is power and selfishness that complete his corrupt leadership.
Power is the corrupting element in this novel. It is an allegory on human morality, so what is said about the animals is implied as part of human nature.