The Devil ultimately wished for despair.
For despair wears down the mind and drains one's will to go forward. When one feels there's nothing left to go for, their soul falls down to Hell, the Devil's domain. This is why the Devil finds it more entertaining to watch people fall to despair, rather than to simply kill them. We describe it as "selling one's soul to the Devil."
When a marksman received a gun from the Devil, the Devil proposed a childish contract: The last bullet would puncture the head of his beloved. The moment he heard that, he sought and shot all the people he loved. Then he told the Devil, "This magical bullet can truly hit anyone just like you say."
With the last bullet gone for good, the hunter traveled to numerous worlds. Sometimes he helped those in need or fought evil, but it was all done on impulse, not a gesture of goodwill or ideals. In one world, he was called a righteous hunter, while in another, he was a bloodstained gunman.
One day, the marksman realized the Devil no longer followed him. He pondered why, then realized that his soul had already fallen to Hell from the beginning. The contract had already been fulfilled, so the Devil had left.
The marksman, now a devil himself, continues to pull the trigger to gather the souls of others. Just as the Devil said, the bullets will puncture anything you please. Forever.
