Ram Raj was a Raj based on Chaturvarnya. As a king, Rama was bound to maintain Chaturvarnya. It was his duty therefore to kill Shambuka, the Shudra, who had transgressed his class and wanted to be a Brahmin. This is the reason why Rama killed Shambuka. But this also shows that penal sanction is necessary for the maintenance of Chaturvarnya. Not only penal sanction is necessary, but penalty of death is necessary. That is why Rama did not inflict on Shambuka a lesser punishment.
