In Animal Farm, was it a good thing that Boxer was told about the van's true purpose?
When Boxer is injured and unable to contribute to the farm, Napoleon makes a secret deal to sell him to a knacker, who will slaughter him and use his parts for glue and other things. Napoleon then lies to the other animals that Boxer is being sent to a hospital. However, the writing on the van side gives the plot away and Clover warns Boxer:
It was uncertain whether Boxer had understood what Clover had said. But a moment later his face disappeared from the window and there was the sound of a tremendous drumming of hoofs inside the van. He was trying to kick his way out.
(Orwell, Animal Farm, george-orwell.org)
Although the animal's intent in warning Boxer was good, they fail to rescue him, and Boxer is too weak to kick out. The net effect was probably bad; Napoleon's lie about Boxer's death in a hospital is treated with suspicion and Boxer himself likely spent his last hours in terrible fear and anger. In this sense, it might have been a cruel mercy to let Boxer believe that he was being anesthetized for medical care rather than being gassed to death.
It certainly was a good thing that Boxer's companions tried to warn him of his impending death. If Boxer had managed to escape from the knacker's van then, atleast the other animals would have protected him from Napoleon's plans to finish him off and would have cared for him till he died a natural death after his retirement:"If he made a good recovery, he might expect to live another three years, and he looked forward to the peaceful days that he would spend in the corner of the big pasture."