My house is being painted and all my books have been removed from the shelves but I did manage to dig out my battered ROTK with my trusty flashlight.
Aragorn said naught in answer, but he took the other's eye and held it, and for a moment they strove thus; but soon, though Aragorn did not stir or move hand to weapon, the other quailed and gave back as if menaced with a blow. "I am a herald and an ambassador, and may not be assailed!"
"Where such laws hold," said Gandalf, "it is also the custom for ambassadors to use less insolence. But no one has threatened you. You have naught to fear from us, until your errand is done. But unless your master has come to new wisdom, then with all his servants you will be in great peril!"
Nobody lays hand on the Mouth, they parley and he rides away, then the battle begins but Aragorn doesn't slay him even then. The narrative switches to Pippin's POV, and he wishes that HE could have killed the Mouth.
Seriously though, didnt the Spartan's kill any messenger that did not bring good news?