That you might wound an animal and it would scurry away and suffer a long and cruel death.
I'm going slightly off-topic, but one of my pet peeves in movies is how instantly everyone dies:
Touch by a sword? Death is instantaneous.
Pierced by an arrow? You're dead before you hit the ground.
Wearing armor? It won't help. At all. One touch and you're done for.
In real life, death usually takes longer. And battlefields (especially ancient battle fields) are filled with screaming as people slowly die.
Its a moral question that every hunter should ask themselves before the hunt and right before you pull the trigger. Months of practice and honing your skill can result in a clean kill. Or buck fever can make you jerk a shot clean over into the next county.