Forgiveness is something to be earned, not given
The duel--or rather, sparing--didn't end with the Chief Guard.
After that, the event continued with the elite soldiers, with the human facing several demons at once. Again, there was always a worried expression on his face, but the fluidity of his movement was nothing but veteran material.
For the soldiers of greed that never actually brought their skill to war before, it was an eye-opening experience. Though they hadn't completely gotten rid of their fear and apprehension toward the sacred weapon, they grew to respect the one wielding such a deadly tool. Especially when they recalled how the human scolded that scary weapon like he would a child.