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

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.