Boxer’s death is a devastating moment in Animal Farm because it is the result of a cruel deception, and because Orwell had long established Boxer as the hardest working, strongest, and most dedicated member of the farm.
Boxer’s joint mottos of “I will work harder!” and “Napoleon is always right!” show his strict dedication to the cause of communism that reflected the attitudes of many members of the working class. His work ethic inspires those around him and he seems the model citizen in their new utopia. Boxer is promised retirement in a field with Benjamin the donkey, but it never comes to pass. As soon as he passes his usefulness to the cause, he is sold for glue. This is the harsh truth of Marxism: it commodifies individuals as well as fields, goods, services, and government systems. It bases value on production, and as Boxer could no longer produce, he no longer had value.
The working class may have rallied for communism, deposed the old order, and asked for the hammer and sickle, but it is exactly the working class that suffers the most under communism. This is what Orwell hoped to communicate through Boxer’s death.