This is a great question, as Animal Farm can be read as an allegory of what was taking place in Russia in the early 1900s to World War II.
The best place to start is to identify Napoleon as Joseph Stalin. If we do this, then we can begin to answer the question of what it means that the pigs were becoming more human.
At first, Joseph Stalin seemed sincere. He really thought that a revolution was necessary for the benefit of his people and country. It might be worth noting that he was born into poverty. However, when he gained power, things began to change. He became the dictator of the Union of Soviet Socialist Republics from 1929 to 1953, and he increasingly introduced a reign of terror, and millions of his own people died. For example, he forced the collectivization of land whereby he took over agriculture. Many farmers refused and they were killed. Moreover, widespread famine occurred that brought immense suffering. Finally, he also began to ally himself with the United States and Britain in World War II.
From this perspective, instead of helping his people by liberating them, he subjugated them further. In this sense, one ruler was replaced with another ruler. To put it in the context of the book, Napoleon was worse than Mr. Jones. Likewise, Stalin was worse than the tzars. He became what he said he was delivering his people from.