Shloka 5.53.2
लाङ्गूलदाह-पर्यटनम् (The Burning Tail and the Parade through Laṅkā)
सम्यगुक्तं हि भवता दूतवध्या विगर्हिता।अवश्यं तु वधादन्यः क्रियतामस्य निग्रहः।।5.53.2।।
samyag uktaṃ hi bhavatā dūtavadhyā vigarhitā | avaśyaṃ tu vadhād anyaḥ kriyatām asya nigrahaḥ ||5.53.2||
What you have said is indeed right: the killing of an envoy is condemned. Yet some restraint must certainly be imposed on him—only, let it be a punishment other than death.
"Indeed, you have spoken very well.Killing of emissary is censured (in sastras). Surely we will impose suitable punishment other than killing.
Even in conflict, dharma sets limits: an envoy is protected by convention and śāstric ethics, so punishment must not violate that protected status.
In Laṅkā’s court, after Hanumān is captured as a messenger, the discussion turns to what penalty is permissible; the decision is to punish without executing him.
Respect for maryādā (ethical boundaries of war and diplomacy), expressed as adherence to the rule that a dūta should not be slain.