While it would be easy to point fingers at the power-hungry Napoleon or even Snowball, the true responsibility for Animal Farm's failure lies with all of the characters.
If some of the animals, like the sheep, had been less apt to follow the dictatorial direction that the farm was taking, that would have been one area that could have improved. Likewise, if other animals had acted less like themselves (donkeys=stubborn; horses=slow-minded, loyal laborers) then the plans of the ruling minority would not have succeeded, either.
In the end, the fault for Animal Farm's failure lies with each character -- even the farmer. While students may be tempted to place blame with the animals who were most instrumental in establishing "the rules" and so forth, nothing would have happened if cooperation were not present.
Since 'Animal Farm' is written as both an allegory and fable, the animal characters represent types and could hardly be held responsible for their comportment. For example, the sheep are non-thinking followers by nature in the same way that the pigs are instinctively selfish. Boxer is a true workhorse and the cat is aloof and independent. Muriel the goat and Benjamin the donkey both have a stubborn streak. The animals remain true in character to their representative types.
As an allegory, the story of Animal Farm has already been written out by history. Its story line is an autopsy of sorts of an idealism incapable of being applied. Could anything else have become of the vast Russian Empire other than a reorganization of power by a new oligarchy?
As a fable, Animal Farm could be considered a cautionary tale, warning the reader about the potential of corruption within a democratic state. In a looser interpretation, one could indeed "blame" the animals for their gullibility and lack of critical judgment. The message would be that privilege in democracy also demands responsibility, and in this respect the animals indeed fell dramatically short.