“We’ll have rules!” he cried excitedly. “Lots of rules! Then when anyone breaks ‘em-”
It seems as though the boys, specifically Jack, are only excited about having rules because they can punish whoever breaks them. This will be important later on in the novel as savagery becomes more and more apparent.
It is also ironic for Jack to give the idea of a punishment to a rule breaker considering that he is the first boy to deliberately break a rule that Ralph created.