Orwell was a socialist and believed that we need a revolution.
He fought against the Fascist dictator Franco in the Spanish Civil War to defend the socialist revolution which was happening in that country. Unfortunately Franco won, and the Communist Party played a part in dividing the left and stopping the revolution from happening. Animal Farm describes the betrayal of Stalin and the Communist Party, in not defending ordinary people, but creating a privileged bureaucracy.
In Animal Farm, Orwell is not saying that revolution cannot work because an elite will always come out on top. He is saying that, unless the bosses (be they capitalists - represented by Jones or Stalinists - represented by the pigs) are completely overthrown, we cannot have a successful revolution.
Napoleon in Animal Farm = Stalin. Snowball = Trotsky, who sent birds around neighbouring farms to spread the message of the revolution, i.e. fought for a revolution in more than one country. Trotsky, sadly, was sidelined and "airbrushed out of history" by the Stalinists. He was exiled and was eventually killed in Mexico in 1940 by a member of Stalin's secret police.
I do not think there is a single person to blame - the obvious candidate is Napoleon, but you could argue that the animals were too stupid to see what was happening and overthrow the pigs.
Read the book yourself and make up your own mind!
Also read Homage to Catalonia by Orwell, to put Animal Farm in context. (see source)