The animals wanted to rebel because they clearly remembered the Sixth Commandment said 'No animal shall kill any other animal.' However, when they wanted to read the Commandment again, they saw that it actually said 'No animal shall kill any other animal without cause,' which is something they believed slipped their mind, even though the last two words were added later on. This is how Squeal managed to convince everyone the Commandment was in fact not violated even after the animals were slaughtered.