Anyone subdued enough that you can pull an arrow out of them is subdued enough that you can deliver a coup de grace with a knife or axe. It's demonstrably not an act of mercy to shoot them twice with the same arrow.
It seems pretty clear that it's a demonstration of how badly defeated the enemy is... that the enemy is incapable of putting up the smallest bit of defense.
The nuances of the message being sent may be a mystery to us, but the high-level message seems pretty clear to me. Some of the possible aspects of the message are (1) demonstrating military his military prowess to his tribe (2) demonstrating his bravery to his tribe (3) increasing the shame felt by the enemy as he died (4) demonstrating his military prowess to surviving enemies to demoralize them.
If the primary intent of the message was to emotionally torture an enemy through shame in his dying moments, then contempt and hate would appear to be the primary motivators.
I hate someone so much I shoot him with an arrow now, he's having trouble moving so I realizing I'm out of arrows walk up yank out the arrow (smiling as he screams while trying to crawl away) and shoot him again at point blank range for the lulz.
EDIT: Not trying to endorse this kind of viewpoint at all, or support it but it is an explanation for how you can hate someone so much that you'd shoot them with an arrow twice.
I'm thinking they were trying to avoid downvotes from people who think conceiving of a way someone might have a particular point of view is essentially an endorsement of that point of view.
Do you lack so much empathy that you can't even put into words your perception of another person's malicious or pathological reasoning? Or do you just assume to get into their mind is to share it?
I don't think vmception is necessarily someone who thinks that way, I read it as someone relatively cynical and finding the note superfluous and unnecessary. So the did it work read as being sarcastic to me, but not because he thought downvotes were deserved.
Arrowheads were not particularly rare or valuable. Smaller points can be made in 5-10 minutes by experienced knappers. The shaft and fletching take a bit more work, but any war group would have been constantly producing them.
I always hated recording knapping sites because the thousands of lost or partial pieces that they just didn't care to pick up are a pain to document.
But if an enemy is unable to prevent the victor from pulling an arrow out, it's both more merciful and less risky to the arrow and the victor to deliver a coup de grace with a knife, axe, or club.
If that were the answer, why would anyone have recorded that the event took place? Reusing arrows would be common, and no particular arrow would be memorable enough to know you shot someone twice with it.
Battlefield success was a huge sign of leadership fitness for Plains Indians. Comanche called it "puha" - it was sort of a marker of divine blessing or luck (good medicine) and it meant you could more easily gather more tribesmen for your war party to go on raids against other tribes.
Shooting them twice with the same arrow would be like the modern equivalent of a 360 no scope in Call of Duty, demonstrating your "puha." See also: https://en.wikipedia.org/wiki/Counting_coup
Well, he probably retrieved the arrow and realized the warrior was still alive so shot him again with it. Why put forth the effort to draw another arrow on someone you hate and have contempt for when you have one in your hand? Plus, it probably made a heck of a story to tell his friends. Bragging is a bit of a thing.
