Lewis on retribution, revenge, and mercy

 For Lewis punishment is about retribution,  which is different from revenge. Revenge is not limited by what someone deserves, and typically when people take revenge, the do more to them than what they did to the other person. When you give someone what they deserve, you can't hurt them more than they deserve, and Lewis is open to saying that courts can be merciful and, for a good reason, give people less than they deserve. 

See here. 

Two Australian penologists responded, here, as did Australian philosopher J. J. C. Smart, here, who in spite of famously becoming an act utilitarian, seems to have been a rule utilitarian at this point. Lewis responded here.