In what way is the sale and eventual slaughter of Boxer at the hands of the knacker a moving experience for the reader?

In Orwell's Animal Farm, one of the
most prominent horses in the story is a male cart horse named Boxer. In the first
chapter of the novel, Major predicts that Boxer will be sold to the knacker and boiled
down and ultimately that is what eventually
happens.

Boxer's departure, though, is a sad one for
Orwell's readers because Boxer and Clover are arguably the most noble creatures in
Animal Farm. Orwell tells us that they were the "most faithful
disciples" of the pigs. They passed on the pigs' teachings to the other animals,
attended all the meetings at the barn, and always "led the singing of 'Beasts of
England'."

As for Boxer, he was an incredibly hard worker
and would continue laboring vigorously in the face of any obstacles. His personal motto
was, "I will work harder!" Eventually, Boxer adds a second motto: "Napoleon is always
right", even though it has become obvious to everyone that Napoleon is a brutal and
ruthless dictator.

In Chapter 4, during the battle with the
humans, Boxer is upset with himself when he accidentally kills one of the humans. In
Chapter 8, Boxer helps rally the animals when the humans make another attack on them.
During that attack, Boxer is wounded and the windmill that he had labored so hard to
help build is destroyed.

Still, Boxer's only desire is to
rebuild the windmill before he retired. Unfortunately, Boxer's tired body was not strong
enough to see that task accomplished. When Boxer fell ill and was being taken away by
the knacker, all the animals were horrified. Although Squealer did not really mean these
words, I think that modern readers of the novel would
agree,

"It was
the most affecting sight I have ever
seen!"

Boxer, the most noble
animal, the backbone of their revolution and labor, had been mercilessly
slaughtered.