Corruption of Ideals
The way the pigs abandon their principles reveals Orwell's central concern about power. As a socialist himself, Orwell believed the Russian Revolution had been hijacked by power-hungry individuals like Stalin.
Initially, Snowball leads the revolution and develops "Animalism" (the farm's ideology). Napoleon seems less important at first, but secretly trains dogs as his enforcers and eventually drives Snowball away. The original values of equality and solidarity become twisted into tools for Napoleon to maintain control.
Orwell doesn't just blame the leaders, though. The other animals share responsibility through their passivity, fear and ignorance. They continue to support Napoleon despite evidence their lives are worsening, and they accept Squealer's propaganda without question.
Remember: Orwell's warning extends beyond politics to any situation where people blindly follow authority or accept propaganda without critical thinking.