"Vibhishana, a tiger among demons, asked Ravana to give up his decision as it was a forbidden course according to the science of royal polity. [5-58-146]
सुमहत्यपराधेऽपि दूतस्यातुलविक्रमः | विरूपकरणं दृष्टं न वधोऽस्तीति शास्त्रतः || ५-५८-१४८
'O Ravana of immeasurable courage even if the emissary has done any kind of harm he may be mutilated but not murdered. Nowhere do the sastras allow killing (of an ambassador). [5-58-148]