I think the purpose of the penal system is to protect society from those who prey on the common citizenry, and "penalty" has noting to do with it.
I am not interested in punishment, retribution or revenge. If society is adequately protected by isolating criminals from the society, then that is adequate and moral.
Clarence Darrow maintained that the persons who committed "real" crimes, such as rape, arson, or murder, were sick. They should be hospitalized and quarantined until they were well, then released. They should not be released before they were well.
The death penalty should be reserved for those persons who are still a danger to society, even incarcerated. Some terrorists perhaps. Maybe lawyers and politicians.