It is very interesting how the animals struggle to get away from the corrupt behavior of human beings and fight so hard for their independence only to have corruption and greed rise up from their own ranks.
The animals suffer in part because Napoleon is becoming more like a human, taking on the bad habits of humans, like drinking whiskey and sleeping in a bed and making deals with other farmers, and he builds an army of ferocious attack puppies (dogs) to protect his interests.
They also suffer because they turn on each other. Napoleon runs Snowball off the farm because they can't agree on how to run the farm. So he is chased off by the attack dogs.
Typically jealousy and suspicion lead Napoleon to believe that Snowball has somehow sabotaged the windmill and that other animals have conspired along with him, again Napoleon.
The savage behavior that Napoleon engages in is representative of a typical dictator, who sacrifices some of his people to teach others a lesson. It is his greed and desire for unlimited power that lead to the innocent deaths of the animals.
Good question! Unfortunately for the animals, it is not an either/or situation. They suffer because Napoleon is a tyrant, but also because of their own weaknesses. If they thought more clearly, they would not accept his rule, or Snowball's arguments. Their weaknesses, and their prior conditions of ignorance and servitude, work together with Napoleon's rule to create a tragedy.
Very good question.
In reality, neither are the primary cause for the animal's suffering in the story. While Napoleon has obviously molded himself into a tyrant worse than any human the animals have dealt with before their revolution, alot of the problems are only allowed to continue do to the animal's own weaknesses. Granted the attack dogs are a deterrent, but they've dealt with armed farmers, what had stopped them from rising up again? Boxer's death should've been the most obvious wake up call.
For example, take the charactere Benjamin. He's lived a long time, and its no secret that he takes no sort of enthusiasm from any promises made by either Snowball or Napoleon. Unlike alot of the other animals, he has an insight that allows him to think outside of the 'herd'. The chapter with Boxer's departure is the first time he ever really does anything close to disent, yet he's known the whole time pretty much what the pigs are up to. Its his own stubborness that keeps him from reading the changing commandments when he is one of the few who can read as well.
You also have more naive characters like Boxer and Clover, who really want whats best for the farm, and are the very sort of hard working people someone can manipulate. The sheep don't think for themselves, and its almost as if the animals are in denial, which could be way they so easily dismiss their own suspicions.
So I would say it would be both.