There are a few key morals in the book, but the one that stands out the most is Lord Acton's famous notion that absolute power corrupts absolutely. The fuller quote is even more insightful.
“Power tends to corrupt and absolute power corrupts absolutely. Great men are almost always bad men, even when they exercise influence and not authority; still more when you superadd the tendency of the certainty of corruption by authority.”
We see that these words are true increasingly in Animal Farm. At first, the pigs start off just like the other animals; they want freedom and tout equality. The concept of animalism is fair. As the story progresses, the pigs, especially Napoleon and Squealer, become more corrupt, as they grow in influence. They use the animals for their own gain. The most prominent example of corruption and self-serving is what they do to Boxer. They sell off Boxer to the knackers, when he can no longer work. They consume his labor, and now they consume what remains of him, his body, for their own purposes and gain.
Finally, the story ends when the pigs turn into men, which shows the corruption is complete. Within the internal logic of the story, man is the epitome of corruption.