At the same time some people do change. Some crimes are circumstantial and people regret and repent. Sometimes people change to be a better person in repentance for the crime. Some people reform for the better. By punishing them now we could shatter someones faith in reformation and repentance. When I think of such situations I always think of Angulimala and Xena.
It is a Catch-22. I say treat each situation as it comes. No matter what there should be a trial. Prosecution can present the extent of the crima and why it must go punished. Defense can defend the circumstance and prove repentance and reformation. The judge/jury can decide if a punishment is warrant. Sometimes prosecutors will also agree to charge with a lesser sentence.