Who abuses power, and who do they represent?

Both Mr. Jones and Napoleon abuse power and subject animals to unspeakable cruelty. Mr. Jones—a drunk farmer running the Manor Farm—represents Tsar Nicholas II, a dictator who was ousted by the Russian Revolution in 1917. As the novel begins, Mr. Jones make animals overwork and sells them soon as they outlive their usefulness. Animals value Old Major's lecture on rebellion so highly that it becomes a cornerstone of their entire “Animalism” philosophy. Later, Mr. Jones and his men are banished from the farm and Napoleon becomes a self-proclaimed leader.
Napoleon symbolizes Joseph Stalin, a Russian ruler, who prioritized his own agendas over the struggles of the common citizens. In George Orwell’s Animal Farm, it is remembered that the primary idea of overthrowing humans was to gain freedom. However, this dream of achieving a world where all animals are equally important is shattered under Napoleon's leadership. Just like humans, Napoleon and his fellow pigs exploit and mistreat the rest of animals. What is more, Napoleon uses his right-hand man, Squealer to justify all the actions that go against the principles of “Animalism.” He rules with an iron fist and eliminates any animal deemed a threat to his leadership.
Further, Napoleon lacks a sense of morality as a leader. A prime example is when he sells the ailing horse—Boxer—to be slaughtered. Pigs engage in activities such as drinking alcohol, sleeping in beds, eating apples, wearing human clothes and trading, which are contrary to their code of conduct. At the end of the novel, pigs have significantly changed to an extent that the other animals cannot differentiate them from humans.

The pigs, particularly Napoleon, abuse their power. Napoleon represents Josef Stalin, the "communist" leader who ruled Soviet Russia as a totalitarian dictator. The other pigs represent the small power elite that coalesced around Stalin.
Like Stalin and his cohort, the pigs betray the ideals of their revolution. The initial idea of the animal revolt, expressed by Old Major, the boar standing for Karl Marx, was that, having gotten rid of the human owners, all the animals would be equal. Having seized the farm, they would use its profits to make life materially better for all the animals. Further, the animals would not behave like the humans. They would not live in houses, wear clothes, sleep in beds, or drink alcohol.
Instead of being true to these ideals, the pigs use trained dogs to terrorize the other animals. The pig stage trials and executions, gradually take away all the rights of the other animals, and commandeer all the farm's profits for themselves. The pigs live in a house, wear clothes, sleep in beds, and act in all ways like humans. The other animals end up worse than they were before.