Boxer's cruel death is a result of Napoleon's tyrannical rule. Though some animals recognize that they are living under tyranny, why they do not act?
There are several reasons why the animals do not act against Napoleon's tyranny. First, the had always lived under tyranny. Jones never allowed the animals any freedom and they were not used to thinking for themselves. That is why the accept the pigs' leadership in the first place. Secondly, Napoleon and Squealer keep reminding the animals of what life was like under Jones, They are afraid is they rebel, Jones or another human may return and they will go back to human rule. Squealer is very good at justifying the actions of the pigs. Finally, Napoleon rules with an iron hand. His dogs are fierce and many animals are afraid of being torn apart. Napoleon has already given them a demonstration of the dog's strength when he holds a meeting and has animals "confess". After they confess, they are torn apart so the animals know the punishment for not being loyal to Napoleon.
One only needs to look at history and at present day societies that embrace socialism as the solution to problems. People are weak; they want someone to "take care of me."
As response #1 so aptly puts, "they are afraid..." Orwell recognizes that there are two kinds in this world: the weak and the strong. Given power, as Golding points out in "Lord of the Flies," the strong become savage and others then fear them.