In media dealing with the Middle Ages, enemy messengers are often killed by brutal rulers in a display of defiance. Do we know how common this was and what were the implications?

by penlanach

An example is The Kingdom of Heaven. Another example although not medieval is the opening of Gladiator.

'Send the messengers head back' seems to be recurring theme in film/tv generally dealing with medieval Europe, or fictional societies inspired by it.

Was this actually common? What were the social reactions to such an act if it is recorded?

mikedash

There is certainly more to say, of course, but u/cthulhushrugged went into some detail on this topic in their award-winning post on the advancing Mongol empire and its envoys, here:

"I am a governor of a village at the height of Genghis Khan's empire. What should I expect to happen after I submit peacefully to Mongol rule?"

It would be well worth checking out this response while you wait to other answers to your query.