In "Animal Farm," what does Napoleon tell Boxer that proves Napoleon ruthless?

Napoleon tells Boxer that he is sending the hard working horse to a rest home when, in fact, he is has sold Boxer to a glue factory where he will be slaughtered. This is obviously ruthless because Boxer has been such an integral part of Animal Farm and the revolution. Boxer is the hardest worker of the animals, to the point that he works himself to exhaustion. Boxer was excessively loyal to Napoleon and followed him without question. To betray Boxer by sending him to a grim death in a slaughterhouse can only be considered cold-hearted and ruthless. However, this mirrors the way many workers in the Soviet Union were treated during the reign of Josef Stalin and that is the point George Orwell, the author of the book, wants to make.