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.
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 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.
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