Napoleon helped to overthrow the humans that had been oppressing the animals in the farm with help from fellow pig Snowball but soon becomes extremely corrupt and cruel - disposing of animals that displease him and adopting policies almost identical to those used by the humans they had fought against.
In the end of the story Napoleon and his fellow pigs (minus Snowball, who ran afoul of Napoleon and disposed of) are so corrupt that the other animals can no longer tell the difference between the pigs and the humans, Napoleon is destroyed and exploded inside the windmill to his death.
In the animated movie he was killed by the rebelling animals. In the live-action version of the book, he is destroyed in the birds flying fastest to dead, the statue of Napoleon's memories is moving out and collasped, he is not the king anymore.
In The Looney Tunes short Wholly Smoke, The Bull Knocked Him out of the drugstore After Porky Bought a Plate
Napoleon was based on real life Soviet dictator Joseph Stalin.