Humans can understand the reason for pain and the need to adapt. Animals cannot be comforted or reasoned with when they are in pain, they just suffer and suffer more. Humans know that they will receive pain meds when they complain of the pain, prosthetic limbs when the stump is healed to make it easier to "adapt", and they can understand the need for all of this.
A chick? All it will understand is that it is in pain, the pain is bigger when the human chops off it's leg and now it is unbalanced and disadvantaged until it, or even IF it, can learn to hop on one leg.
All that in the name of compassion and humanity? Please kill the poor thing.