Why is Clover confused at the beginning of Chapter 8?

This question refers to Animal Farm.

At the beginning of Chapter 8, Clover is confused because she believes that one of the commandments established by the animals has been broken. She remembers that the sixth commandment established that no animal shall be killed by another animal. As animals have indeed been killed, she is confused as to how to resolve this.

Therefore, she asks Muriel to read her the sixth commandment again. The commandment reads "No animal shall kill another animal without cause." Although Clover did not remember the last words, she thinks no more of it. However, we know that the last words have been added by Napoleon in order to justify his killings.