Think about it this way: what, in your opinion, would have happened to an animal that had mentioned this in the hearing of the pigs or the dogs?
Look at what has happened so far. The dogs have been (at the order of the pigs) killing all sorts of animals for whatever reason. The pigs seem to be making up reasons or having animals killed for no reason. In that sort of a situation, what would you do and what would you think?
To me, I think I would be scared stiff. I would be thinking that if I mentioned this commandment, the pigs and dogs would get mad at me and I would be the next one to be killed. So I think that the animals were terrified. They did not want to call attention to themselves and make it look like they were rebelling. That is why they did not point out that the killings went against the commandments.
It is critical to know that at that point of time, Napoleon had become more power-hungry and ambitious, so he would kill anyone who hinders his rule. So, by using the dogs, the pigs were threatened and coerced to confess to their falsified accusations against them (paralleled to the show trials in Russia), and killed as a warning to other animals. But, the animals thought that they have recalled one of the commandment, saying that:
"no animal shall kill any other animal"
But, the animals were scared to voice out their opinions, in fear of being killed by the dogs. They were in a state of terror, and dared not say that the killings had indeed revoked one of the commandments. They did not want to suffer the fate of the dogs and the hens, so they did not say anything, as they did not want Napoleon to think that they were rebelling against his rule, and suffer the same punishment for their misdeeds.