After Boxer is taken from the farm to be killed, Napoleon creates a fantasy story about his death. The other animals initially believe the sign on the van that reads "Horse Slaughterer," but are swayed by Squealer, who tells them a sad tale of Boxer's final days in the hospital:
"...And at the end, almost too weak to speak, he whispered in my ear that his sole sorrow was to have passed on before the windmill was finished. 'Forward, comrades!' he whispered. 'Forward in the name of the Rebellion. Long live Animal Farm! Long live Comrade Napoleon! Napoleon is always right.'"
(Orwell, Animal Farm, george-orwell.org)
Squealer shamelessly uses the dead Boxer as a propaganda tool, claiming that he wanted the other animals to trust in Napoleon over all other things. Boxer cannot refute his words, and the other animals are so used to trusting Napoleon and the pigs that they believe Squealer when he tells them that the knacker's van was purchased by the hospital, but never painted over. This shows Napoleon's cruel nature, as he outright sells Boxer to be killed without anyone's knowledge, and then lies about it to raise his own status with the other animals.