Why did no one "care to mention" recalling a commandment about animals killing other animals in the hearing of the pigs or the dogs?
The dogs gather animals that had annoyed Napoleon into the yard for an inquisition. The hens that had formed a mutiny to protest the taking of their eggs, the porkers who dared question Napoleon on the cancelling of Sunday meetings, and even poor Boxer. The purpose of this hearing was to make them confess their traitorous acts. Once they have done this, the dogs leap forward and rip out their throats. All except Boxer, who is too strong to be taken down.
Other animals then step forward and "confess" crimes and are quickly put to death. The confessions have clearly been coerced, and therefore the animals on the farm realize that should they speak up about the commandment against killing, they will be killed themselves.