Throughout Animal Farm, "I will work harder" remains one of Boxer's personal creeds. He constantly pushes himself to greater and greater lengths to contribute his labor to the farm. Unfortunately, he weakens as he grows older. He should probably have slowed down, but he continued to declare: "I will work harder." Eventually, he collapses from overwork. The pigs claim they will send him to see a veterinarian, but they actually sell him to a knacker--someone who slaughters horses to be used as pet food or glue, among other things.
Boxer's fate is tragic because he is one of Animal Farm's most sympathetic characters. He could not contribute to the intellectual development of Animalism, but he was its most loyal follower, and he joyfully makes personal sacrifices (such as working extra hours during his free time) to contribute to its success. It is tragic that his reward for such virtue is betrayal.