Strong punishment should fit the crime and not the criminal.
No criminal is a criminal before committing a crime. The crimes committed by a criminal will vary from time to time, both in terms of number and severity. A criminal may start committing more heinous crimes or may start reforming himself/herself and commit minor crimes. The key point is that the severity of crime will keep on varying. This means that a punishment cannot fit the criminal for the criminal is only as good or as bad as the crime he/she commits. In short, the reputation of a criminal is variable. The punishment then should not fit the criminal. This would be like punishing the criminal for his past crimes.
For example, a serial petty theft offender might commit a murder. Now, in this case the punishment ought to be severe. The punishment must be in proportion to the crime committed ie. murder. It should not be that the murderer is given a lenient punishment because he has only committed thefts in the past, which will be the case if the punishment fits the criminal.