Whenever I read accounts of the killing of emissaries (such as the Greeks killing the Persian emissaries, or the Khwarezmians, Mamluks, Hungarians, etc, killing Mongol emissaries), I wonder "why did they do that?"

It seems like the cons of killing a foreign power's emissaries are pretty clear -- it gives a belligerent country a causus belli if they didn't have one, and might even provoke a nation inclined toward continued diplomatic talks toward a war.

What were the pros? I suppose that if they thought that the emissary was a spy who was gathering data on their fortifications' weak points, granary supplies, etc, then they might want him not to get back to his masters, and I also suppose that if an emissary is suspected of talking to people other than the sovereign and subverting their loyalty (e.g., offering one of the Hungarian king's prominent discontented vassals some deal if he'll side with the Mongols), that could be a problem. Overall, though, it seems like escorting the emissary and making sure he only talks to the sovereign or his representatives and doesn't acquire intelligence or talk to any potential fifth columnists seems safer than just executing him.

Were there more positives to executing an emissary than I see? Was just really cathartic to kill an arrogant foreign diplomat and yell "THIS. IS. SPARTAAAAA!"? (Or, perhaps, "THIS. IS. KHWAREZM!" or whatever the local equivalent was.)