Feng had used a special type of magic to enchant the bullets, which he shot towards the scumbags. The bullets had the special properties of fire, once impaled inside someone hot energy will exclude out of the bullet and will burn the body into cinders. He did that because the body will turn to ashes and a zombie will not be formed.
As for the reason why he let the young boy live, the reason was simple. He had heard the regrets filled voice of the boy and saw the disgust the boy hold for his life. Seeing that, he remembered the past of his son. He understood that the boy wasn't evil, rather society had veered off the peaceful path, and cruelty has taken an unjust place in society. In this world, to survive in a society as such, the weak can only discard basic morals and principal to survive. Due to which, for survival, the boy has chosen the path of evil.
